Great refracting telescope, presented by the emperor Alexander I.
It is a colourless, highly refracting liquid, boiling at 78°; it fumes on exposure to moist air.
In it are, moreover, enclosed unicellular glands pouring their highly refracting contents, of a more or less rod-like shape, directly to the exterior.
The more highly organized species have often very numerous eyes (Amphiporus, Drepanophorus), which are provided with a spherical refracting anterior portion, with a cellular " vitreous body," with a layer of delicate radially arranged rods, with an outer sheath of dark pigment, and with a separate nerve-twig each, springing from a common or double pair of branches which leave the brain as n.
Besides these more highly differentiated organs of vision, more primitive eyes are present in others down to simple stellate pigment specks without any refracting apparatus.
Such an instrument consists of a triangular prism set with its refracting edge vertical on a rigid platform attached to a massive stand.
These are of dense flint-glass (Schott 0.102), and each has a refracting angle of 63° 29'.
On the other hand, that the direction of complete polarization should be independent of the refracting power of the matter composing the cloud has been considered mysterious.
The crystals are feebly doubly refracting, and in polarized light exhibit a banded structure parallel to the cube faces.
Lehmann it melts at 168° (or at a slightly lower temperature in its water of crystallization) and on cooling forms optically isotropic crystals; at 125.6° the mass becomes doubly refracting, and from a solution rhombohedral (optically uniaxial) crystals are deposited; by further cooling acicular rhombic crystals are produced at 82.8°, and at 32.4° other rhombic forms are obtained, identical with the product obtained by crystallizing at ordinary temperatures.
In their natural condition the marekanite spheres are doubly refracting, but when they have been heated and very slowly cooled they lose this property and no longer exhibit any tendency to sudden disintegration.
An admissible error of phase of 4X will correspond to an error of IX in a reflecting and 2X in a (glass) refracting surface, the incidence in both cases being perpendicular.
It is a colourless, strongly refracting liquid, which boils at about 220° C., slight decomposition setting in above 150° C. Water decomposes it with production of leucone.
Benzene is a colourless, limpid, highly refracting liquid, having a pleasing and characteristic odour.
If p" is the refracting angle of the prism, and n the magnifying power of the eye-piece, then p"ln will be the distance observed.
3) refracts upwards, while the prism Q, which has its refracting edge perpendicular to that of P, refracts towards the right.
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.
This shows that the connexion between the refrangibility of light and its wave-length does not obey any simple law, but depends on the nature of the refracting medium.
This, however, is not the case with ordinary refracting media, the refrangibility generally increasing more and more rapidly as the wave-length diminishes.
In studying the dispersion of the aniline dyes, a prism with a very small refracting angle is made of two glass plates slightly inclined to each other and enclosing a very thin wedge of the dye, which is either melted between the plates, or is in the form of a solution retained in position by surface-tension.
Only very thin layers are sufficiently transparent to show the dispersion near or within an absorption band, and a large refracting angle is not required, the dispersion usually being very considerable.
Rutherfurd devised one made of flint glass with two crown glass compensating prisms; whilst Thallon employed a hollow prism containing carbon bisulphide also compensated by flint glass prisms. In direct vision spectroscopes the refracting prisms and slit are in the observing telescope.
There are three refracting angles possible, one of Ito° between two adjacent prism faces, one of 60° between two alternate prism faces, and one of 90° between a prism face and the base.
Mariotte explained the inner halo as being due to refraction through a pair of alternate faces, since the minimum deviation of an ice-prism whose refracting angle is 60° is about 22°.
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.
In Germany and elsewhere refracting theodolites and transit instruments are sometimes employed.
The substance is usually optically isotropic, though sometimes it exhibits anomalous double refraction; fibrous zinc sulphide which is doubly refracting is to be referred to the hexagonal FIG.
When in 1666 he made his discovery of the different refrangibility of light of different colours, he soon perceived that the faults of the refracting telescope were due much more to this cause than to the spherical figure of the lenses.
Prop. 3) "that all refracting substances diverged the prismatic colours in a constant proportion to their mean refraction"; and he drew the natural conclusion "that refraction could not be produced without colour," and therefore "that no improvement could be expected from the refracting telescope" (Treatise on Optics, p. 112).
The first person who succeeded in making achromatic refracting telescopes seems to have been Chester Moor Hall, a gentleman of Essex.
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.'
Like Gregory and Hall, he argued that, since the various humours of the human eye were so combined as to produce a perfect image, it should be possible by suitable combinations of lenses of different refracting media to construct a perfect object-glass.
We have thus followed somewhat minutely the history of the gradual process by which Dollond arrived independently at his invention of the refracting telescope, because it has been asserted that he borrowed the idea from others.
The whole history of his researches proves how fully he was aware of the conditions necessary for the attainment of achromatism in refracting telescopes, and he may be well excused if he so long placed implicit reliance on the accuracy of experiments made by so illustrious a philosopher as Newton.
Refracting Telescope In its simplest form the telescope consists of a convexobjective capable of forming an imageof a distant object and.
In spite of the improvements in the manufacture of optical glass practically the same crown and flint glasses as used by John Dollond in 1758 for achromatic objectives are still used for all the largest of the modern refracting telescopes.
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.
The largest refracting telescope in active use is the Yerkes telescope, with an object-glass of 40-in.
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.
Subsequent papers by Dulong were concerned with " New determinations of the proportions of water and the density of certain elastic fluids " (1820, with Berzelius); the property possessed by certain metals of facilitating the combination of gases (1823 with Thenard); the refracting powers of gases (1826); and the specific heats of gases (1829).
Consequently the monochromatic class includes the aberrations at reflecting surfaces of any coloured light, and at refracting surfaces of monochromatic or light of single wave length.
It is seen (ignoring exceptional cases) that the pencil does not meet the refracting or reflecting surface at right angles; therefore it is astigmatic (Gr.
27) that the wind exerts a very marked influence in refracting sound-beams. From 1868 Henry continued to be annually chosen as president of the National Academy of Sciences; and he was also president of the Philosophical Society of Washington from the date of its organization in 1871.
In the Copepoda the median eye may undergo considerable elaboration, and refracting lenses and other accessory structures may be developed in connexion with it.
This body appears to be the source of light, and has behind it a reflector formed of concentric lamellae, while, in front, in some cases, there is a refracting lens.
Tive than the first, is an ellipse having the luminous point for a focus, and its centre at the foot of the perpendicular from the luminous point to the refracting 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.
Differences of refractive index produce their greatest dispersive effects when incidence on the refracting surface is nearly tangential.
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.
The high excellence of the s sle data collected by them was a combined result of their skill, and of the vast improvement in refracting telescopes due to the genius of Joseph Fraunhofer (1787-1826).
That this is not a necessary characteristic of light was discovered by Christian Huygens, who found that, whereas a stream of sunlight in traversing a rhomb of spar in any but one direction always gives rise to two streams of equal brightness, each of these emergent streams is divided by a second rhomb into two portions having a relative intensity dependent upon the position with respect to one another of the principal planes of the faces of entry into the rhombs - the planes through the axes of the crystals perpendicular to the refracting surfaces.
Examining the light reflected from the windows of the Luxemburg palace with a doubly refracting prism, he was led to infer (though more refined experiments have shown that this is not strictly the case) that light reflected at a certain angle, called the polarizing angle, from the surface of transparent substances has the same properties with respect to the plane of incidence as those of the ordinary stream in Iceland spar with respect to the principal plane of the crystal.
Malus that the interposition of a doubly refracting plate between a polarizer and an analyser regulated for extinction has the effect of partially restoring the light, and he used this property to discover double refraction in cases in which the separation of the two refracted streams was too slight to be directly detected.
The phenomena of chromatic polarization afford a ready means of detecting doubly refracting structure in cases, such as that produced in isotropic bodies by strain, in which its effects are very minute.
In the contrary case, total reflection commences as soon as sin i =µ 1, µ being still the relative refractive index of the more highly refracting medium; and for greater angles of incidence r becomes imaginary.
Applying this interpretation to the formulae given above, it follows that when the incident light is polarized at an azimuth a to the plane of incidence and the second medium is the less refracting, the reflected light at angles of incidence exceeding the critical angle is elliptically polarized with a difference of phase A between the components polarized in the principal azimuths that is given by tan (A/2) =cot i l l (1 - µ 2 cosec 2 i).
Thus if A = 42, p. must exceed 7r/8 or 2.414, that is, the substance must be at least as highly refracting as a diamond: if A =7 /4, µ must be greater than 37r/16 or 1.4966, and when this is the case, it is possible by two reflections to convert into a circularly polarized stream a beam of light polarized at 45° to the plane of incidence.
504), who has found the remarkable result that copper, gold, magnesium and silver have refractive indices less than unity, and this has been completely confirmed by observations with metallic prisms of small refracting angle.
This is a plate made of two equal wedges of quartz, that can be moved over one another so as to vary its thickness, and are cut so that the faces of the plate are parallel to the optic axis, which in the first wedge is perpendicular and in the second is parallel to the refracting edge.
In most cases refracting and reflecting systems are arranged so that the natural interpupillary distance is reduced.