Rays Sentence Examples

If any rays missed their bodies, the sand reflected it to them.

Half way up the steep was a yawning cave, black as night beyond the point where the rainbow rays of the colored suns reached into it.

Below, the house was bathed in the first rays of morning sun.

They seemed to walk for hours, until the first rays of morning lightened the forest.

The hidden sun was setting, and the white snow clouds glowed eerily, lit by the last rays of light.

The rays of the sun fell upon the trees, so that the twigs sparkled like diamonds and dropped in showers when we touched them.

We barely slept that night, in eager anticipation of glimpsing our first rays of sunshine!

Speak of the sun and you see its rays! and she smiled amiably at Pierre.

In either case, narrow, secondary rays are formed at intervals, just as in the stem.

And by some latent sequence of thought the descent of the Mozhaysk hill, the carts with the wounded, the ringing bells, the slanting rays of the sun, and the songs of the cavalrymen vividly recurred to his mind.

Soil temperature is partly dependent on the direct rays of the sun, partly on the color and constitution of the soil, and partly on the water content of the soil.

Remaining itself in repose, it rays out, as it were, from its own fullness an image of itself, which is called vas, and which constitutes the system of ideas of the intelligible world.

The insects are guided by light, being very sensitive to ultra-violet rays, and also by scent and hearing.

In this all-important doctrine of the Sephiroth, the Kabbalah insists upon the fact that these potencies are not creations of the En Soph, which would be a diminution of strength; that they form among themselves and with the En Soph a strict unity, and simply represent different aspects of the same being, just as the different rays which proceed from the light, and which appear different things to the eye, are only different manifestations of one and the same light; that for this reason they all alike partake of the perfections of the En Soph; and that as emanations from the Infinite, the Sephiroth are infinite and perfect like the En Soph, and yet constitute the first finite things.

Prisms deflect rays of light towards their bases.

The rays of this star spend close upon a century in travelling hither.

And that this is the intire and adequate cause of their colours, is manifest, because they have no power to change or alter the colours of any sort of Rays incident apart, but put on all colours indifferently, with which they are inlightened.

These rays come to a focus at a point F lying in the focal plane of the telescope.

The cones of rays issuing from a point situated only a little to the FIG.

A similar six-rayed system of cracks, bisecting the angles between the rays of the previous set, is produced when a blunt punch is gradually pressed against a sheet of mica; this is known as the "pressure figure."

It is equal to the actual diameter of the cylinder of rays admitted by a telescope.

Theory and experiment alike prove that a double line, of which the components are equally strong, is better resolved when, for example, one-sixth of the horizontal aperture is blocked off by a central screen; or the rays quite at the centre may be allowed to pass, while others a little farther removed are blocked off.

The extreme discrepancy is that between the waves which travel through the outermost parts of the object-glass at L and L'; so that if we adopt the above standard of resolution, the question is where must P be situated in order that the relative retardation of the rays PL and PL' may on their arrival at B amount to a wave-length (X).

The question is whether, when the adjustment of focus is correct for the central rays of the spectrum, the error of phase for the most extreme rays (which it is necessary to consider) amounts to a quarter of a wave-length.

If 8 and 4' denote the angles with the normal made by the incident and diffracted rays, the formula (5) still holds, and, if the deviation be reckoned from the direction of the regularly reflected rays, it is expressed as before by (0+0), and is a minimum when 8 = 0, that is, when the diffracted rays return upon the course of the incident rays.

Then the relative retardation of the extreme rays (corresponding to the edges A, B of the grating) is mnX.

If BQ be the direction for the first minimum (the darkness between the central and first lateral band), the relative retardation of the extreme rays is (mn+1)X.

The irregularity of spacing has thus the effect of a convex lens, which accelerates the marginal relatively to the central rays.

It may be remarked that these calculations apply to the rays in the primary plane only.

The experiment is easily made on a laboratory scale, with a small source of light, the rays from which, in their course towards a rather distant screen, are disturbed by the neighbourhood of a heated body.

The proportionality of the secondary disturbance to sin 43 is common to the present law and to that given by Stokes, but here there is no dependence upon the angle 0 between the primary and secondary rays.

Among the Greeks and Romans various speculations as to the cause of the how were indulged in; Aristotle, in his Meteors, erroneously ascribes it to the reflection of the sun's rays by the rain; Seneca adopted the same view.

So far we have only considered rays of homogeneous light, and it remains to investigate how lights of varying refrangibilities will be transmitted.

Taking the refractive index of water for the red rays as 0;, and for the violet rays as 1 r, we can calculate the following values for the minimum deviations corresponding to certain assigned values of n.

To this point we have only considered rays passing through a principal section of the drop; in nature, however, the rays impinge at every point of the surface facing the sun.

It may be readily deduced that the directions of minimum deviation for a pencil of parallel rays lie on the surface of cones, the semi-vertical angles of which are equal to the values given in the above table.

This was first suggested by Thomas Young, who showed that the rays producing the bows consisted of two systems, which, although emerging in parallel directions, traversed different paths in the drop. Destructive interference between these superposed rays will therefore occur, and, instead of a continuous maximum illumination in the direction of minimum deviation, we should expect to find alternations of brightness and darkness.

Marine rainbow is the name given to the chromatic displays formed by the sun's rays falling on the spray drawn up by the wind playing on the surface of an agitated sea.

They are formed by parallel rays of light emanating from two sources, as, for example, the sun and its image in a sheet of water, which is situated between the observer and the sun.

The action of the sun's rays stimulates the cells of the skin to increase the pigment as a protection to the underlying tissues, e.g.

Among the revisions may be adduced some addition to our knowledge of dyspepsia, attained by analytic investigations into the contents of the stomach at various stages of digestion, and by examining the passage of opaque substances through the primae vine by the Rntgen rays.

The discovery of the Rntgen rays has also extended the physician's power of vision, as in cases of aortic aneurysm, and other thoracic diseases.

The temperature required in the fusion of sheet-glass and of other glasses produced in tank furnaces is much lower than that attained in steel furnaces, and it is consequently pos Since the discovery of the Rntgen rays, experiments have been made to ascertain the effects of the different constituents of glass on the transparency of glass to X-rays.

The oxides of lead, barium, zinc and antimony are found perceptibly to retard the rays.

The glass tubes, therefore, from which the X-ray bulbs are to be fashioned, must not contain any of these oxides, whereas the glass used for making the funnel-shaped shields, which direct the rays upon the patient and at the same time protect the hands of the operator from the action of the rays, must contain a large proportion of lead.

This elaborate type of scolex appears to be an adaptation to grasp the spiral intestinal valve of sharks and rays.

His work won him the Rumford medal of the Royal Society in 1838, and in 1843 he received its Royal medal for a paper on the "Transparency of the Atmosphere and the Laws of Extinction of the Sun's Rays passing through it."

About the same time Francesco Maurolico, or Maurolycus, the eminent mathematician of Messina, in his Theore y nata de Lumine et Umbra, written in 1521, fully investigated the optical problems connected with vision and the passage of rays of light through small apertures with and without lenses, and made great advances in this direction over his predecessors.

He discloses as a great secret the use of a concave speculum in front of the aperture, to collect the rays passing through it, when the images will be seen reversed, but by prolonging them beyond the centre they would be seen larger and unreversed.

All objects, therefore, which lie beyond a certain point (the conjugate focus of the dioptric system of the eye, the far point) are indistinctly seen; rays from them have not the necessary divergence to be focused in the retina, but may obtain it by the interposition of suitable concave lenses.

Hence in its condition of repose such an eye cannot distinctly see parallel rays from a distance and, still less, divergent rays from a near object.

The word actinometer is now usually applied to instruments for measuring the actinic or chemical effect of luminous rays; their action generally depends upon photochemical changes.

In KiushiO, Shikoku and the southern half of the main island, the months of July and August alone are marked by oppressive heat at the sea-level, while in elevated districts a cool and even bracing temperature may always be found, though the direct rays of the sun retain distressing power.

The majority of snakes are active during the day, their energy increasing with the increasing temperature; whilst some delight in the moist sweltering heat of dense tropical vegetation, others expose themselves to the fiercest rays of the midday sun.

When the rays of the sun or a candle, or dark radiation from a warm body, are incident on the vanes, the dark side of each vane is repelled more than the bright side, and thus the vanes are set into rotation with accelerated speed, which becomes uniform when the forces produced by the radiation are balanced by the friction of the pivot and of the residual air in the globe.

Savary expresses preference for this second plan, and makes the pertinent remark that in both these models " the rays of red light in the two solar images will be next to each other, which will render the sun's disk more easy to be observed than the violet ones."

The chief objections to the method are that, as one star is in the axis of the telescope and the other displaced from it, the images are not both in focus of the eye-piece,3 and the rays from the two stars do not make the same angle with the optical axis of each segment.

Again, in an ocular heliometer by Steinheil double image is similarly produced by a divided prism of total reflection placed in parallel rays.

In the last the field is full of false light, and it is not possible to give sufficiently minute and steady separation to the images; and there are of necessity a collimator, two prisms of total reflection, and a small telescope through which the rays must pass; consequently there is great loss of light.

Bone has shown that when exposed for some time to the sun's rays it undergoes certain polymerization changes which lead to the deposition of a film of heavy hydrocarbons on the surface of the tube.

It may be regarded as an epicycloid in which the rolling and fixed circles are equal in diameter, as the inverse of a parabola for its focus, or as the caustic produced by the reflection at a spherical surface of rays emanating from a point on the circumference.

Hence all rays between =0 will be confined in the space between the outer dome and a circle of radius OP cos 0, and the weakening of intensity will be chiefly due to vertical spreading.

Hence sound rays, in passing from one medium into another, are bent in towards the normal, or the reverse, according as the velocity of propagation in the former exceeds or falls short of that in the latter.

It further follows, as in the analogous case of light, that there is a certain angle termed the critical angle, whose sine is found by dividing the less by the greater velocity, such that all rays of sound meeting the surface separating two different bodies will not pass onward, but suffer total reflection back into the first body, if the.

Hence, rays of sound proceeding from a distant source, and therefore nearly parallel to each other,.

Any sound (such as that of the human voice) transmitting its rays into the reflector, and communicating vibratory motion to the membrane, will cause the feather to trace a sinuous line on the paper.

Giotto's zodiac at Padua was remarkable (in its undisturbed condition) for the arrangement of the signs so as to be struck in turns, during the corresponding months, by the sun's rays.'

It is particularly noteworthy from the phosphorescence which it exhibits when heated, or after exposure to the sun's rays; hence its synonym "Canton's phosphorus," after John Canton (1718-1772), an English natural philosopher.

If the surrounding aether is thereby disturbed, the waves of light arriving from the stars will partake of its movement; the ascertained phenomena of the astronomical aberration of light show that the rays travel to the observer, across this disturbed aether near the earth, in straight lines.

As, however, our terrestrial optical apparatus is now all in motion along with the matter, we must dealt .with the rays relative to the moving system, and to these also Fermat's principle clearly applies; thus V+ (lu'--mv'-Fnw') is here the velocity of radiation in the direction of the ray, but relative to the moving material system.

This theory secures that the times of passage of the rays shall be independent of the motion of the system, only up to the first order of the ratio of its velocity to that of radiation.

The most fundamental experimental confirmation that the theory of the aether has received on the optical side in recent years has been the verification of Maxwell's proposition that radiation exerts mechanical force on a material system, on which it falls, which may be represented in all cases as the resultant of pressures operating along the rays, and of intensity equal at each point of free space to the density of radiant energy.

In September 1839 a 3-foot speculum was finished and mounted on an altazimuth stand similar to Herschel's; but, though the definition of the images was good (except that the diffraction at the joints of the speculum caused minute rays in the case of a very bright star), and its peculiar skeleton form allowed the speculum to follow atmospheric changes of temperature very quickly, Lord Rosse decided to cast a solid 3-foot speculum.

Dispersion is therefore due to the fact that rays of different colours possess different refrangibilities.

As the light is twice refracted, the dispersion is increased, and the rays, after transmission through the prism, form a divergent system, which may be allowed to fall on a sheet of white paper, forming the wellknown solar spectrum.

This confirmed his previous conclusion that the rays increase in refrangibility from red to violet.

In order to find the amount of dispersion caused by any given prism, the deviations produced by it on two rays of any definite pure colours may be measured.

The angle of difference between these deviations is called the dispersion for those rays.

If 6 F and Sc are the angular deviations of these rays, then S F - Sc is called the mean dispersion of the prism.

If the refracting angleof the prism is small, then the ratio of the dispersion to the mean deviation of the two rays is the dispersive power of the material of the prism.

P. Leroux discovered that iodine vapour refracted the red rays more than the violet, the intermediate colours not being transmitted; and in 1870 Christiansen found that an alcoholic solution of fuchsine refracted the violet less than the red, the order of the successive colours being violet, red, orange, yellow; the green being absorbed and a dark interval occurring between the violet and red.

Some of the slightly cloudy Ceylon sapphires, usually of greyish-blue colour, display when cut with a convex face a chatoyant luminosity, sometimes forming a luminous star of six rays, whence they are called "star sapphires".

If 1 2 and t l are thicknesses traversed by the extreme rays, t = t 2 - t,, and if, as is usually the case, the prism is filled right up to its refraction cap, = o, and t becomes equal to the greatest thickness of the medium which is made use of.

These rays are apparently the trajectories of positively charged particles having masses of the order of magnitude of the gaseous molecules.

It should be mentioned that the infra-red rays have a remarkable damping effect on the phenomena of phosphorescence, a fact which has.

He mentioned as an important exception the case of ferric ferrocyanide, which, when dissolved in oxalic acid, transmits the rays in great abundance, though the same rays be absorbed both by ferrocyanides and by ferric salts.

Soret has confirmed, for the ultra-violet rays, Dr Gladstone's conclusions with regard to the identity of the absorption spectra of different chromates.

The fact that benzene and its derivatives are remarkable for their powerful absorption of the most refrangible rays, and for some characteristic absorption bands appearing on dilution, led Hartley to a more extended examination of some of the more complicated organic substances.

At the other end of the collimator there is a condensing lens for bringing the rays into parallelism.

In Browning's form the setting is automatic. The dispersion may be further increased by causing the rays to pass more than once through the prism or prisms. Thus, by means of a system of reflecting prisms, Hilger passed the dispersed rays six times through one prism, and, by similar means, Browning passed the rays first through the upper part of a train and then back through the lower part.

The light is rich in the violet and ultra-violet rays, and consequently is employed in photography.

But, though illuminated by the rays of art, and loaded with the exuberant panegyrics of humanists and poets, the reign of the first Medicean pontiff, by its unbounded devotion to purely secular tendencies and its comparative neglect of the Church herself proved disastrous for the See of St Peter.

He developed a great research laboratory of experimental physics, attracting numerous workers from many countries and colonies; advances were made in the investigation of the conduction of electricity through gases, in the determination of the charge and mass of the electron and in the development of analysis by means of positive rays.

In this event, the whiteness of the tips will be due to the scattering or irregular reflexion of the incident rays of light from the surface of the numerous gas bubbles.

Since the minimum deviation is least for the least refrangible rays, it follows that the red rays will be the least refracted, and the violet the more refracted, and therefore the halo will be coloured red on the inside.

As the sun rises, the rays enter the prisms more and more obliquely, and the angle of minimum deviation increases; but since the emergent ray makes the same angle with the refracting edge as the incident ray, it follows that the parhelia will remain on the parhelic circle, while receding from the inner halo.

The anthelion (a) may be explained as caused by two internal reflections of the solar rays by a hexagonal lamellar crystal, having its axis horizontal and one of the diagonals of its base vertical.

The emerging rays are parallel to their original direction and form a colourless image on the parhelic circle opposite the sun.

The two achromatic lenses, C and D, bring the rays to a focus on the plane surface of the large lens, E, forming an image there.

From the large lens, E, the rays pass through the open air for a considerable distance, depending upon how much the mast has been raised, to the lower optical system.

Horizontal rays of light entering at the top were reflected by a prism down the tube and focussed on to a sheet of paper in front of the helmsman inside the submarine.

The top is closed by a pressure-tight window, inside of which is a prism which reflects the light rays vertically down the tube to a prism at the bottom end, where they are reflected in a horizontal direction and focussed in an eyepiece attached to the bottom of the tube.

The Order of Pius was founded in 1847 by Pius IX.; there are now three classes; the badge is an eight-pointed blue star with golden flames between the rays, a white centre bears the founder's name; the ribbon is blue with two red stripes at each border.

The badge is a red rayed cross with gold rays in the angles, in the centre a representation of the pillars of Hercules; the cross is attached to the yellow and white ribbon by a green laurel wreath.

There are five classes; the badge is a silver sun of seven clustered rays, with crescent and star between each cluster; on a gold centre is the sultan's name in black Turkish lettering, surrounded by a red fillet inscribed with the words Zeal, Devotion, Loyalty; it is suspended from a red crescent and star; the ribbon is red with green borders.

The badge is a gold sun with seven gold-bordered green rays; the red centre bears the crescent, and it is also suspended from a gold crescent and star; the ribbon is green bordered with red.

The cross of white and gold clustered rays bears in a blue centre a silver star-shaped mirror.

The badge has an elaborate design; it consists of a star of purple, red, yellow, gold and silver rays, on which are displayed old Japanese weapons, banners and shields in various coloured enamels, the whole surmounted by a golden kite with outstretched wings.

There, a sun which never sets sends feeble rays that maintain a low equable temperature, rarely rising more than a few degrees above the freezing-point.

After passing through the slit the diverging rays fall upon the 8 in.

The rays, rendered parallel by the collimator objective, meet a plane mirror (f) of silvered glass, which reflects them to the prisms (g, g').

On such a slope effectual draining is easily accomplished, and the greatest possible benefit is derived from the sun's rays.

But as the rays of light, even in passing through transparent glass, lose much of their energy, which is further weakened in proportion to the distance it has to travel, the nearer the plant can be placed to the glass the more perfectly will its functions be performed; hence the importance of constructing the roofs at such an angle as will admit the most light, especially sunlight, at the time it is most required.

They require only such shade as will shut out the direct rays of the sun, and, though abundant moisture must be supplied, the atmosphere should not be overloaded with it.

The eye end of the telescope tube is removed - a counterpoise to the object end being substituted in its place - and a prism is inserted at the intersection of the visual axis with the transit axis, so that the rays from the object-glass may be reflected through one of the tubes of the transit axis to an eye-piece in the pivot of this tube.

It is very transparent for Rntgen rays, whereas paste imitations are opaque.

The electrical resistance is about that of ordinary glass, and is diminished by one-half during exposure by Rntgen rays; the dielectric constant (16) is greater than that which should correspond to the specific gravity.

Becquerel's observation in 1896 that certain uranium preparations emitted a radiation resembling the X rays observed by Rntgen in 1895.

Like the X rays, the Becquerel rays are invisible; they both traverse thin sheets of glass or metal, and cannot be refracted; moreover, they both ionize gases, i.e.

Notwithstanding, these resemblances, these two sets of rays are not indentical.

The Becquerel rays have a marked chemical action on certain substances.

Bordas, C. Doelter and others, that the rays induce important changes in the colours of many minerals.

The different varieties of rays used are controlled by the intervention of screens or filtering substances, such as silver, lead or aluminium.

The flat surface is spread to allow the maximum amount of sunlight to fall upon it, as it is by the absorption of energy from the sun's rays by means of the chlorophyll contained in the cells of the leaf that the building up of plant food is rendered possible; this process is known as photo-synthesis; the first stage is the combination of carbon dioxide, absorbed from the air taken in through the stomata into the living cells of the leaf, with water which is brought into the leaf by the wood-vessels.

The rays had to struggle through a disturbing medium; they reached him refracted, dulled and discoloured by the thick gloom which had settled on his soul, and, though they might be sufficiently clear to guide him, were too dim to cheer him.

Frequently an arc or band is visibly composed of innumerable short rays separated by distinctly less luminous intervals.

These rays are more or less perpendicular to the arc or band; sometimes they are very approximately parallel to one another, on other occasions they converge towards a paint.

Longer rays often show an independent existence.

Not infrequently rays extend from the upper edge of an arc towards the zenith.

Combinations of rays sometimes resemble a luminous fan, or a series of fans, or part of a hollow luminous cylinder.

Rays often alter suddenly in length, seeming to stretch down towards the horizon or mount towards the zenith.

The rays may lie very close together, or may be widely separated from one another.

Arcs, bands and, generally speaking, the more regular and persistent forms, show their greatest frequencies earlier in the night than rays or patches.

But clearly, whilst the arcs and bands, and to a lesser extent the patches, showed a marked preference for the magnetic meridian, the rays showed no such preference.

Regular arcs were selected in most cases, but the lowest height obtained was for a collection of rays forming a curtain which was actually situated between the two stations.

Such extremely bright auroras seem very rare, however, even in the Arctic. There is a general tendency for both bands and rays to appear brightest at their lowest parts; arcs seldom appear as bright at their summits as nearer the horizon.

Birkeland (19) supposes the ultimate cause to be cathode rays emanating from the sun; C. Nordmann (24) replaces the cathode rays by Hertzian waves; while Svante Arrhenius (25) believes that negatively charged particles are driven through the sun's atmosphere by the Maxwell-Bartoli repulsion of light and reach the earth's atmosphere.

Cathode rays usually have a velocity about a tenth that of light, but in exceptional cases it may approach a third of that of light.

On either Birkeland's or Nordmann's theory, the electric impulse from the sun acts indirectly by creating secondary cathode rays in the earth's atmosphere, or ionizing it so that discharges due to natural differences of potential are immensely facilitated.

Percussionfigures, readily made on the cleavage-faces, have rays parallel to faces of the rhombic dodecahedron; whilst figures etched with water represent the four-faced cube.

In Ulva and Mesocarpus the chromatophore is a single plate, which in the latter genus places its edge towards the incident light; in Spirogyra they are spiral bands embedded in the primordial utricle; in Zygnema they are a pair of stellate masses, the rays of which branch peripherally; in Oedogonium they are longitudinally-disposed anastomosing bands; in Desmids plates with irregular margins; in Cladophora polyhedral plates; in Vaucheria minute elliptical bodies occurring in immense numbers.

And we will be ever suffusing such a one with the rays of our loving thought.

This conversion, represented as having been brought about while he was hunting on Good Friday by a miraculous appearance of a stag bearing between his horns a cross or crucifix surrounded with rays of light, has frequently been made the subject of artistic treatment.

The medullary rays extend radially from the centre of the tree to the bark at right angles to the grain of the wood, and serve during life to bind the whole together as well as to convey nourishment from one part of the tree to another.

In oak this develops the beautiful silver grain by cutting longitudinally through the medullary rays.

No medullary rays are visible; the wood is straight in the grain, durable, strong and elastic, easy to work, and is used by the carpenter for internal and external constructional work, and by the joiner for his fittings.

The wood is very hard, tough, with fine regular grain and close texture, the annular rings being distinct and the medullary rays well marked.

When it is cut along these rays beautiful markings are revealed, called silver grain.

The Persian Gulf has an unenviable reputation for its dangers from heat-stroke, and the sun's rays seem to have a peculiar deadly power in this region, for the risk of exposure is greater than in any part of the world, though other countries have a temperature which is equally high.

The clear atmosphere is in its upper strata free from clouds and dust, so that the sun's rays undergo scarcely any absorption and strike down with full force on the light-brown desert soil, from which they are radiated and reflected to a great extent.

The relative humidity of the air along the shores of the Gulf is high, so that exposure to the direct and reflected rays of the sun and radiation from the hot soil are encountered in a moist atmosphere.

It is easily seen that if the mirror be rotated at the same angular velocity as the sun the right ascensions will remain equal throughout the day, and therefore this device reflects the rays in the direction of the earth's axis; a second fixed mirror reflects them in any other fixed direction.

Foucault's heliostat reflects the rays horizontally in any required direction.

By adjusting the right ascension of the plane ABC and rotating the axis with the angular velocity of the sun, it follows that BC will be the direction of the solar rays throughout the day.

X is the mirror rotating about the point E, and placed so that (if EB is the horizontal direction in which the rays are to be reflected) (I) the normal CE to the mirror is jointed to BC at C and is equal in length to BE, (2) the rod DBC passes through a slot in a rod ED fixed to, and in the plane of, the mirror.

Silbermann's heliostat reflects the rays in any direction.

It is easy to show that rays falling on the mirror in the direction BC will be reflected along BD.

The mirror mm is attached to the framework pafe, the members of which are parallel to the incident and reflected rays SO, OR, and the diagonal pf is perpendicular to the mirror.

The arc y r is set so as to reflect the rays in the required direction.

Fizeau he carried on a series of investigations on the intensity of the light of the sun, as compared with that of carbon in the electric arc, and of lime in the flame of the oxyhydrogen blowpipe; on the interference of heat rays, and of light rays differing greatly in lengths of path; and on the chromatic polarization of light.

The bleaching-powder casks must be kept in a dry place, as cool as possible, and never exposed to the direct rays of the sun, in order to prevent a decomposition which now and then has even led to explosions.

The hot season begins officially in the Punjab on the 15th of March, and from that date there is a steady rise in the temperature, induced by the fiery rays of the sun upon the baking earth, until the break of the rains in June.

For it is easy to understand by the canons above mentioned that the greatest objects may appear exceedingly small, and the contrary, also that the most remote objects may appear just at hand, and the converse; for we can give such figures to transparent bodies, and dispose them in such order with respect to the eye and the objects, that the rays shall be refracted and bent towards any place we please, so that we shall see the object near at hand or at any distance under any angle we please.

He argued that the different humours of the human eye so refract rays of light as to produce an image on the retina which is free from colour, and he reasonably argued that it might be possible to produce a like result by combining lenses composed of different refracting media.'

Adopting a hypothetical law of the dispersion of differently coloured rays of light, he proved analytically the possibility of constructing an achromatic object-glass composed of lenses of glass and water.

When the axis of the eye-lens coincides with that of the object-glass, and the focal point of the eye-lens is coincident with the principal focus of the object-lens, parallel rays.

Also the diameter of the pencil or parallel rays emerging from the eye-lens to the diameter of the object-lens inversely as, the magnifying power of the telescope.

Hence one of the best methods of determining the magnifying power of a telescope to measure the diameter of the emergent pencil of rays, after the FIG.

If we desire to, utilize all the parallel rays which fall upon an object-glass it is necessary that the full pencil of emerging rays should enter the observer's eye.

For normal eyes the natural adaptation is not to focus for quiteparallel rays, but on objects at a moderate distance, and practically, therefore, most persons do adjust the focus of a telescope, for most distinct and easy vision, so that the rays emerge from the eye-piece very slightly divergent.

Hence it is clear that if the two positive lenses of equal curvature power of o 60 and 0.102 respectively are combined with a negative lens of light flint o 569, then a triple objective, having no secondary spectrum (at any rate with respect to the blue rays), may be obtained.

Parallel rays falling on A A converge at F, where an image is formed; the rays are then reflected from B and converge at P, where a second and more enlarged image is formed.

The chromatic aberration of the object-glass of one of these telescopes is corrected for photographic rays, and the image formed by it is received on a highly sensitive photographic plate.

The other telescope is corrected for visual rays and its image is formed on the plane of the spider-lines of a filar micrometer.

Prism i is in the axis of the declination circle and always reflects rays along?

An excellent feature is the short distance between the eye-piece and the declination axis, so that 1 In the bent telescope refracting prisms are employed at the corners to change the direction of the rays.

This framework is provided with guides on which the platform, whilst preserving its horizontality, is V the observer has to follow the eye-end in a comparatively small circle; another good point is the flattening of the cast-iron centrepiece of the tube so that the flange of the declination axis is attached as near to the axis of the telescope tube as is consistent with free passage of the cone of rays from the object-glass.

This latter carries a hyperboloidal mirror, which returns the rays towards the centre of the large mirror and causes them to converge less rapidly.

The reflected rays pass down the tube from the direction of the elevated pole instead of upward towards that pole.

The difficulty is that the automatic motion of a single mirror capable of reflecting the rays of any star continuously along the axis of a fixed horizontal telescope, requires a rather complex mechanism owing to the variation of the angle of reflexion with the diurnal motion.

The largest refracting telescope yet made, viz., that constructed by Gautier for the Paris exhibition of 1900, was arranged on this plan (type F), the stars' rays being reflected along the horizontal axis re rac or of a telescope provided with visual and with photo graphic object-glasses of 49-in.

Therefore, to observe stars of a different declination it will be necessary either to shift the direction of the fixed telescope, keeping its axis still pointed to the coelostat mirror, or to employ a second mirror to reflect the rays from the coelostat mirror along the axis of a fixed telescope.

Within the focus of the object-glass is a right-angled prism of total reflection, which diverts the converging rays from the object-glass at right angles to the axis of the telescope, and permits the observing micrometer n to be mounted in the very convenient position shown in the figure.

As soon divorce the investigation of the shape and material of a mirror from the laws of the incidence of the rays that form images in it, and call it a science of reflection!

But a deviation, termed diffraction, does occur, and consequently the complete theory of shadows involves considerations based on the nature of the rays themselves; this aspect is treated in Diffraction Of Light.

It is characterized by the presence of a special development of the lophioderm or median fin-system, namely, the ventral fin, which is composed of two portions, a lower keel-like portion, which underlies an upper chambered portion, each chamber containing typically a pair of gelatinous fin rays.

For the preparation of " artist's oil," the finest form of linseed oil, the refined oil is placed in shallow trays covered with glass, and exposed to the action of the sun's rays.

Practically all the best vineyards (which are grown on flat terraces on the slopes, and not on the slopes themselves) face south-west and so get the full benefit of the sun's rays.

C. Rntgen of Munich made in 1896 his remarkable discovery of the so-called X or Rntgen rays, a class of radiation produced by the impact of the cathode particles against an impervious metallic screen or anticathode placed in the vacuum tube.

The study of Rntgen rays was ardently pursued by the principal physicists in Europe during the years 1897 and 1898 and subsequently.

The principal property of these Rntgen rays which attracted public attention was their power of passing through many solid bodies and affecting a photographic plate.

The astonishing feat of photographing the bones of the living animal within the tissues soon rendered the Rontgen rays indispensable in surgery and directed an army of investigators to their study.

She was with him, too, during his earlier Caspian campaigns, and was obliged on this occasion to shear off her beautiful hair and wear a close-fitting fur cap to protect her from the rays of the sun.

As the freshets begin to lessen and retire into the deeper channels, the currents form natural embankments on their edges, preventing the return of a small portion of water which is thus left stagnant on the sands, and exposed to the action of the sun's rays.

The Gaussian theory, however, is only true so long as the angles made by all rays with the optical axis (the symmetrical axis of the system) are infinitely small, i.e.

If S (fig.5) be any optical system, rays proceeding from an axis point 0 under an angle u l will unite in the axis point O'1; and those under an angle 24 2 in the axis point 0'2.

If the object point be infinitely distant, all rays received by the first member of the system are parallel, and their intersections, after traversing the system, vary according to their " perpendicular height of incidence," i.e.

The course of the rays in the meridional section is no longer symmetrical to the principal ray of the pencil; and on an intercepting plane there appears, instead of a luminous point, a patch of light, not symmetrical about a point, and often exhibiting a resemblance to a comet having its tail directed towards or away from the axis.

If now the image be sufficiently sharp, inasmuch as the rays proceeding from every object point meet in an image point of satisfactory exactitude, it may happen that the image is distorted, i.e.

This ray, named by Abbe a " principal ray " (not to be confused with the " principal rays " of the Gaussian theory), passes through the centre of the entrance pupil before the first refraction, and the centre of the exit pupil after the last refraction.

From this it follows that correctness of drawing depends solely upon the principal rays; and is independent of the sharpness or curvature of the image field.

The rays with an angle of aperture smaller than u* would not have the same distance of intersection and the same sine ratio; these deviations are called "zones," and the constructor endeavours to reduce these to a minimum.

The most important is the chromatic difference of aberration of the axis point, which is still present to disturb the image, after par-axial rays of different colours are united by an appropriate combination of glasses.

Considerable advances in our knowledge of the various chromogenic bacteria have been made by the studies of Beyerinck, Lankester, Engelmann, Ewart and others, and have assumed exceptional importance owing to the discovery that Bacteriopurpurin - the red colouring matter contained in certain sulphur bacteria - absorbs certain rays of solar energy, and enables the organism to utilize the energy for its own life-purposes.

The lower area of bactericidal action extends much farther to the right, because the quartz allows more ultra-violet rays to pass than does glass.

The observations Bacteria g 5' of Downes and Blunt in 1877 left it uncertain whether the bactericidal effects in broth cultures exposed to solar rays were due to thermal action or not.

The practical effect of the bactericidal action of solar light is the destruction of enormous quantities of germs in rivers, the atmosphere and other exposed situations, and experiments have shown that it is especially the pathogenic bacteria - anthrax, typhoid, &c. - which thus succumb to lightaction; the discovery that the electric arc is very rich in bactericidal rays led to the hope that it could be used for disinfecting purposes in hospitals, but mechanical difficulties intervene.

The recent application of the action of bactericidal rays to the cure of lupus is, however, an extension of the same discovery.

The purple bacteria have thus two sources of energy, one by the oxidation of sulphur and another by the absorption of " dark rays."

The timber bears a striking resemblance to that of the oak, which has been mistaken for chestnut; but it may be distinguished by the numerous fine medullary rays.

The mineral also phosphoresces, under the Rntgen rays.

The poppy cultivated in Asia Minor is the variety glabrum, distinguished by the sub-globular shape of the capsule and by the stigmata or rays at the top of the fruit being ten or twelve in number.

He taught, previous to the Polish physicist Witclo, that vision does not result from the emission of rays from the eye, and wrote also on the refraction of light, especially on atmospheric refraction, showing, e.g.

His experiments were especially concerned with the power of transmitting dark heat possessed by various substances and with the changes produced in the heat rays by passage through different materials.

In optics, the term caustic is given to the envelope of luminous rays after reflection or refraction; in the first case the envelope is termed a catacaustic, in the second a diacaustic. Catacaustics are to be observed as bright curves when light is allowed to fall upon a polished riband of steel, such as a watch-spring, placed on a table, and by varying the form of the spring and moving the source of light, a variety of patterns may be obtained.

The simplest case of a caustic curve is when the reflecting surface is a circle, and the luminous rays emanate from a point on the circumference.

When the rays are parallel, the reflecting surface 1 Elie Bocthor (1784-1821) was a French orientalist of Coptic origin.

The Cartesian equation to the caustic produced by reflection at a circle of rays diverging from any point was obtained by Joseph Louis Lagrange; it may be expressed in theform 1(4,2_ a2) (x 2+ y2) - 2a 2 cx - a 2 c 2 1 3 = 2 7 a4c2y2 (x2 + y2 - c2)2, where a is the radius of the reflecting circle, and c the distance of the luminous point from the centre of the circle.

Secondary caustics are orthotomic curves having the reflected or refracted rays as normals, and consequently the proper caustic curve, being the envelope of the normals, is their evolute.

For a circle, when the rays emanate from any point, the secondary caustic is a limacon, and hence the primary caustic is the evolute of this curve.

The simplest instance of a caustic by refraction (or diacaustic) is when luminous rays issuing from a point are refracted at a straight line.

When the refracting curve is a circle and the rays emanate from any point, the locus of the secondary caustic is a Cartesian oval, and the evolute of this curve is the required diacaustic. These curves appear to have been first discussed by Gergonne.

For the caustic by refraction of parallel rays at a circle reference should be made to the memoirs by Arthur Cayley.

In these primeval forests the vegetation is excessively rank; passage has to be forced through thick underwood and creeping plants, between giant trees, whose foliage shuts out the sun's rays; and the land teems with animal and insect life of every form and colour.

Into this chaos enter from time to time broad rays of sunshine, the efforts of a few enlightened monarchs to evolve order from disorder, and to supply to their people the blessings of peace and civilization.

This is due to a stratum of hot air at some distance above the sea level, the rays of light near the horizon being practically horizontal, while those at greater elevations are fairly concave.

It may happen that the change in density is so great that only the upper rays reach the eye; we are then met with the curious illusion of seeing inverted ships in the clouds, although nothing is visible on the ocean.

The pith is encircled by a cylinder of secondary wood, consisting of single or multiple radial rows of tracheids separated by broad medullary rays composed of large parenchymatous cells; the tracheids bear numerous bordered FIG.

The large medullary rays give to the wood a characteristic parenchymatous or lax appearance, which is in marked contrast to the more compact wood of a conifer.

The genus Pinus serves as an illustration of wood of a distinct type characterized by the absence of xylemparenchyma, except such as is associated with the numerous resincanals that occur abundantly in the wood, cortex and medullary rays; the medullary rays are composed of parenchyma and of horizontal tracheids with irregular ingrowths from their walls.

The medullary rays usually consist of a single tier of cells, but in the Pinus type of wood broader medullary rays also occur and are traversed by horizontal resin-canals.

An examination of the wood of branches, stems and roots of the same species or individual usually reveals a fairly wide variation in some of the characters, such as the abundance and size of the medullary rays, the size and arrangement of pits, the presence of wood-parenchyma - characters to which undue importance has often been attached in systematic anatomical work.

When tracheids occur in the medullary rays of the xylem these are replaced in the phloem-region by irregular parenchymatous cells known as albuminous cells.

An anatomical peculiarity in the veins of Pinus and several other genera is the continuity of the medullary rays, which extend as continuous plates from one end of the leaf to the other.

It is of interest to note that the leaves of Gnetum, while typically Dicotyledonous in appearance, possess a Gymnospermous character in the continuous and plate-like medullary rays of their vascular bundles.

The intensity of the light diminishes merely because the total energy, though unaltered, is distributed over a wider and wider surface as the rays diverge from the source.

Thus the atmosphere absorbs a part of the sun's rays, and the greater the distance which the rays have to traverse the greater is the proportion which is absorbed, so that on this account the sun appears less bright towards sunset.

The solution is moderately transparent for a large number of rays in the neighbourhood of the green part of the spectrum; it is, on the whole, much more opaque for red rays, but is readily penetrated by certain red rays belonging to a narrow region of the spectrum.

The lower gaseous cloaks absorb a large part of the light admitted by the photosphere, and especially at the limb and for the more refrangible rays the loss of intensity is very marked.

Consider the rays which meet the eye (at unit distance) at an angle d from the centre of the sun's disk; in their Theory previous passage through the partially translucent por.

Now measures made by Auwers with the Cape heliometer showed no difference, amounting to o I ", and so far negative the idea that the rays reach us after issuing from a level where is sensibly different from unity.

In the solar explanation, the serpent is the darkness driven away by the rays of the sun.

This inferiority of the Gregorian he explained as being probably due to the mutual interference of the rays as they crossed at the principal focus before reflection at the second mirror.

The paper was finally entitled " Theory of Systems of Rays," and the first part was printed in 1828 in the Transactions of the Royal Irish Academy.

This singular result is still known by the name conical refraction," which he proposed for it when he first predicted its existence in the third supplement to his " Systems of Rays," read in 1832.

So that, were a glass so exactly figured as to collect any one sort of rays into one point, it could not collect those also into the same point, which having the same Incidence upon the same Medium are apt to suffer a different refraction.

But these seemed very great difficulties, and I have almost thought them insuperable, when I further considered, that every irregularity in a reflecting superficies makes the rays stray 5 or 6 times more out of their due course, than the like irregularities in a refracting one; so that a much greater curiosity would be here requisite, than in figuring glasses for Refraction.

As the Rays of light differ in degrees of Refrangibility, so they also differ in their disposition to exhibit this or that particular colour.

Colours are not Qualifications of Light, derived from Refractions, or Reflections of natural Bodies (as 'tis generally believed), but original and connate properties, which in divers Rays are divers.

Some Rays are disposed to exhibit a red colour and no other; some a yellow and no other, some a green and no other, and so of the rest.

Nor are there only Rays proper and particular to the more eminent colours, but even to all their intermediate gradations.

And this Analogy 'twixt colours, and refrangibility is very precise and strict; the Rays always either exactly agreeing in both, or proportionally disagreeing in both.

The species of colour, and degree of Refrangibility proper to any particular sort of Rays, is not mutable by Refraction, nor by Reflection from natural bodies, nor by any other cause, that I could yet observe.

When any one sort of Rays hath been well parted from those of other kinds, it hath afterwards obstinately retained its colour, notwithstanding my utmost endeavours to change it.

I have refracted it with Prismes, and reflected it with Bodies, which in Day-light were of other colours; I have intercepted it with the coloured film of Air interceding two compressed plates of glass, transmitted it through coloured Mediums, and through Mediums irradiated with other sorts of Rays, and diversly terminated it; and yet could never produce any new colour out of it.

It would by contracting or dilating become more brisk, or faint, and by the loss of many Rays, in some cases very obscure and dark; but I could never see it changed in specie.

And therefore minium reflecteth Rays of any colour, but most copiously those indued with red; and consequently when illustrated with day-light, that is with all sorts of Rays promiscuously blended, those qualified with red shall abound most in the reflected light, and by their prevalence cause it to appear of that colour.

And for the same reason Bise, reflecting blew most copiously, shall appear blew by the excess of those Rays in its reflected light; and the like of other bodies.

The generative organs arid coelom probably did not send extensions along the rays into the brachioles; but apparently nerves from the aboral centre, after passing through the thecal plates, met in a circumoral ring, from which branches passed into the plate under each main food-groove, and thence supplied the brachioles.

The Silurian genera Eucladia and Euthemon have the rays greatly reduced and merged in the disk, so that the ambulacrals are unseen.

We have in like manner, as derivatives of a given curve, the caustic, catacaustic or diacaustic as the case may be, and the secondary caustic, or curve cutting at right angles the reflected or refracted rays.

If the eye was placed at the focus, no sensation of light was observed, although small pieces of charcoal or blackened platinum foil were immediately raised to incandescence, thus giving rise to visible rays.

The experiment is more easily carried out with the electric light than with sunlight, as the former contains a smaller proportion of visible rays.

Both rock-salt and carbon bisulphide are extremely transparent to the luminous and also to the infra-red rays The iodine in the solution, however, has the property of absorbing the luminous rays, while transmitting the infra-red rays copiously, so that in sufficient thicknesses the solution appears nearly black.

Any small body which was a good absorber of dark rays was rapidly heated to redness when placed at the focus.

Platinized platinum (platinum foil upon which a thin film of platinum had been deposited electrolytically) and charcoal were rendered incandescent, black paper and matches immediately inflamed, ordinary brown paper pierced and burned, while thin white blotting-paper, owing to its transparency to the invisible rays, was scarcely tinged.

A simpler arrangement, also employed by Tyndall, is to cause the rays to be reflected outwards parallel to one another, and to concentrate them by means of a small flask, containing the iodine solution and used as a lens, placed some distance from the camera.

Since the rays used by Tyndall in these experiments are similar to those emitted by a heated body which is not hot enough to be luminous, it might be thought that the radiation, say from a hot kettle, could be concentrated to a focus and employed to render a small body luminous.

For it is easy to see that if, by means of lenses of rock-salt or mirrors, we focused all or nearly all the rays from a small surface on to another surface of equal area, this would not raise the temperature of the second surface above that of the first; and we could not obtain a greater concentration of rays from a large heated surface, since we could not have all parts of the surface simultaneously in focus.

The desired result could be obtained if it were possible, by reflection or otherwise, to cause two different rays to unite without loss and pursue a common path.

Tyndall used the dark rays from a luminous source, which are emitted in a highly concentrated form, so that it was possible to obtain a high temperature, which was, however, much lower than that of the source.

They require only such shade as will shut out the direct rays of the sun, and, though abundant moisture must be supplied, the atmosphere should not be loaded with it.

These fins contain numerous (15-17) rays, a feature in which the fish differs from the Acanthopterygians.

If the plate receives a mixed charge, as, for example, from an induction coil, a "mixed" figure results, consisting of a large red central nucleus, corresponding to the negative charge, surrounded by yellow rays, corresponding to the positive charge.

The problem of measuring from an axis perpendicular to this plane is solved on the principle that the incident and reflected rays of light make equal angles with the perpendicular to a reflecting surface.

Let the parallel dotted lines represent rays of light we regard as infinitely distant, a star for example.

He supposed their tails to result from the action of solar rays, which, in traversing their mass, bore off with them some of their subtler particles to form trains directed away.

Thousands of the dark lines in the solar spectrum agree absolutely in wave-length with the bright rays artificially obtained from known substances, and appertaining to them individually.

Integrated light, accordingly, tells nothing about velocity; but analysed light does, when it includes bright or dark rays the normal positions of which are known.

The usual bands were, however, temporarily effaced in the two brilliant apparitions of 1882 by vivid rays of sodium and iron, emitted during the excitement of perihelion-passage.

The former, if it existed at all, could be found only in the more depressed portions; and even here it would evaporate under the influence of the sun's rays, forming a vapour which, if it existed in considerable quantity, would in some way make itself known to our scrutiny.

The only active cause of such changes is the varying temperature produced by the presence or absence of the sun's rays.

Among his results was that during the progressive phases from before the first quarter till the full moon the heat received increases in a much greater proportion than the light, from which it followed that the former was composed mainly of heat radiated from the moon itself in consequence of the temperature which it assumed under the sun's rays.

Precise conclusions are possible only when a gaseous body is transparent through and through, so that the gas emits its characteristic rays - or when the rays from an incandescent body of any kind pass through a gaseous envelope at a temperature lower than that of the body itself.

In order to isolate a polarized pencil of rays with a rhomb of Iceland spar, it is necessary to have a crystal of such a thickness that the emergent streams are separated, so that one may be stopped by a screen.

All the rays through a given point in the first principal focal plane of the anterior system of lenses traverse the plate as a parallel beam and reunite at the corresponding point of the second focal plane of the posterior system, each in its passage being divided into two by the plate having a given relative retardation.

In the greater part of the Ebro basin the heat of summer is even more intense, The treeless mostly steppe-like valley with a brightcoloured soil acts like a concave mirror in reflecting the suns rays and, moreover, the mountains and highlands by which the valley is enclosed prevent to a large extent the access of winds.

As children of Apollo, they are taken to signify the rays of the sun.

When the shortest distance obtained by the highest strain of accommodation is insufficient to recognize small objects, distinct vision is possible at even a shorter distance by placing a very small diaphragm between the eye and the object, the pencils of rays proceeding from the object-points, which otherwise are limited by the pupils of the eye, being thus restricted by the diaphragm.

If the instrument has a sensible lens diameter, and is arranged so that the centre of rotation of the eye can coincide with the intersection of the principal rays, the lens can then form with the eye a centred system.

The selection of the rays emerging from the lens and actually employed in forming the image is undertaken by the pupil of the eye which, in this case, is consequently the exit pupil of the instrument.

The principal rays, which on the object-side connect the object-points with the centre of the entrance pupil, intersect the axis on the image-side at the centre of rotation M of the eye.

M is therefore the intersection of the principal rays.

If, however, we fix the points lying towards the margin of the field of view, the diaphragm gradually cuts off more and more of the rays which were necessary to fill the pupil, and in consequence the brightness gradually falls off to zero.

In most cases, and also in corrected systems, the intersection of the principal rays is no longer available for the centre of rotation of the eye, and this kind of observation is impossible.

In most cases a diaphragm regulates the rays.

The intersection of the principal rays in this case lies in the middle of the entrance pupil or of the exit pupil.

C and D are the outermost rays which can pass through the instrument.

So long as the pupil of the observer alone undertakes the regulation of the rays there is no perceptible diminution of illumination in comparison with the naked eye vision.

When the pupil regulates the aperture of the rays producing the image the aberrations of the ordinary lenses increase considerably with the magnification, or, what amounts to the same thing, with the increase in the curvature of the surfaces.

Axial aberration is reduced by distributing the refraction between two lenses; and by placing the two lenses farther apart the errors of the pencils of rays proceeding from points lying outside the axis are reduced.

In all microscopes the rays are limited, not in the eyepiece, but in the objective, or before the objective when using a condenser.

The centre of the entrance pupil is the point of intersection of the principal rays; and it is therefore determinative for the perspective representation on the plane focused for.

To ensure the telecentric transmission, the diaphragm in the back focus of the objective may be replaced by a diaphragm in the front focal plane of the condenser, supposing that uniformly illuminated objects are being dealt with; for in this case all the principal rays in the object-space are transmitted parallel to the axis.

Whether the entrance pupil be before or behind the object, in general its position is such that it lies not too near the object, so that the principal rays will have in the object space only trifling inclinations towards one another or are strictly parallel.

The eyepiece, which by means of narrow pencils represents the relatively large real image at infinity, transmits from all points of this real image parallel pencils, whereby the inclination of the principal rays becomes further increased.

In the case of the negative eyepiece, on the other hand, the divergence of the principal rays through the eyepiece is also further augmented, but their point of intersection is not accessible to the eye.

The increase of the inclination of the principal rays, which arises with the microscope, influences the perception of the relief of the object.

The brightnesses of image points in a median section of the pencil are proportional to the aperture of the lens, supposing that the rays are completely reunited.

But since no rays are lost in this transmission (apart from the slight loss due to reflection) the brightness of the image point in the water is as large as that in air, although the apertures have become less.

In immersion-systems a very much greater aggregate of rays is used in the representation than is possible in dry-systems. In addition to a considerable increase in brightness the losses due to reflection are avoided; losses which arise in passing to the back surface of the cover-slip and to the front surface of the front lens.

The rectilinear rays, which we have considered above, but which have no real existence, are nothing but the paths in which the light waves are transmitted.

Further, the different transparencies of the cells for the ultra-violet rays render it unnecessary to dye the preparations.

From the section Regulation of the Rays (above) it is seen that the resolving power is opposed to the depth of definition, which is measured by the reciprocal of the numerical aperture, I/A.

But, owing to the various partial reflections which the illuminating cone of rays undergoes when traversing the surfaces of the lenses, a portion of the light comes again into the preparation, and into the eye of the observer, thus veiling the image.

The extremely small particles of dust (motes in a sunbeam) in the rays are made perceptible by the diffracted light, whilst by ordinary illumination they are invisible.

The same observation can be made with the cone of rays of a reflector, and in the same way the fine rain-drops upon a dark background and the fixed stars in the sky become visible.

Care must here be taken, by using suitable dark-field screens, that no direct rays enter the observing system.