Spectra sentence example
There is clearly no theoretical limit to the resolving power of gratings, even in spectra of given order.
This is the ordinary formula for a reflecting plane grating, and it shows that the spectra are formed in the usual directions.
Attacked in detail, they vanish one after another into as many teasing spectra of uncertainty.
We have now reduced the law for the bands to a form which we have found applicable to a series of lines, but with this important difference that while a in the case of line spectra is a small corrective term, it now forms the constant on which an essential factor in the appearance of the band depends.
The spectra of the stars he obtained by using, outside the object-glass of his telescope, a large prism, through which the light passed to be brought to a focus in front of the eye-piece.Advertisement
He has also shown that the nitrophenols yield, in addition to the colourless true nitrophenol ethers, an isomeric series of coloured unstable quinonoid aci-ethers, which have practically the same colour and yield the same absorption spectra as the coloured metallic salts.
A series of equivalent solutions all containing the same coloured ion have absorption spectra which, when photographed, show identical absorption bands of equal intensity.
By repeated fractionations he was able to divide yttrium into distinct portions which gave different spectra when exposed in a high vacuum to the spark from an induction coil.
The directions of the lateral spectra are such that the passage from one element of the grating to the corresponding point of the next implies a retardation of an integral number of wave-lengths.
In considering the relative brightnesses of the different spectra, it is therefore sufficient to attend merely to the principal directions, provided that the whole deviation be not so great that its cosine differs considerably from unity.Advertisement
The light stopped by the opaque parts of the grating, together with that distributed in the central image and lateral spectra, ought to make up the brightness that would be found in the central image, were all the apertures transparent.
There will be destruction by interference of the first, third and odd spectra generally; while the advantage gained in the spectra of even order is not in dispersion, nor in resolving power, but simply in brilliancy, which is increased four times.
But it is possible that, as suggested by Rowland,' the structure of natural spectra may be too coarse to give opportunity for resolving powers much higher than those now in use.
But a very slight relative displacement will cause the apparition of the odd spectra.
The advantage of approximate bisection lies in the superior brilliancy of the surviving spectra; but in any case the compound grating may be considered to be perfect in the longer interval, and the definition is as good as if the bisection were accurate.Advertisement
The latter found that certain gratings exercised a converging power upon the spectra formed upon one side, and a corresponding diverging power upon the spectra on the other side.
He has also shown how to rule a plane surface with lines so disposed that the grating shall of itself give well-focused spectra.
Rowland to his brilliant invention of concave gratings, by which spectra can be photographed without any further optical appliance.
The use of a grating is very convenient, for not only are there several spectra in view at the same time, but the dispersion can be varied continuously by sloping the grating.
In addition to these wave-lengths there are other groups centred round the wave-lengths which are submultiples of the principal one - the overlapping spectra of the second and higher orders.Advertisement
It is evident that the effect at the focal point is the obliteration of the first and other spectra of odd order, so that as regards the spectrum of the first order we may consider that the two beams interfere.
In this way the principal features of the phenomenon are accounted for, and Schuster has shown further how to extend the results to spectra having their origin in prisms instead of gratings.
In addition to magnesium and sodium the lines of potassium, lithium and also the carbon flutings exhibited in cometary spectra, has been seen.
On the other hand, the theory encounters a very serious difficulty in the fact that all molecules possess a great number of possibilities of internal motion, as is shown by the number of distinct lines in their spectra both of emission and of absorption.
His experiments, in the same year, on the photographic registration of stellar spectra, marked an innovation of a momentous character.Advertisement
If we compare the spectrum produced by refraction in a glass prism with that of a diffraction grating, we find not only that the order of colours is reversed, but also that the same colours do not occupy corresponding lengths on the two spectra, the blue and violet being much more extended in the refraction spectrum.
The refraction spectra for different media also differ amongst themselves.
Edward C. Pickering carried on his study of stellar spectra with the funds of the Henry Draper Memorial at Harvard, endowed by his widow (née Mary Anna Palmer) .
In 1835, in a paper on "The Prismatic Decomposition of Electrical Light," he proved that sparks from different metals give distinctive spectra, which afforded a ready means of discriminating between them.
Bolometric methods may be used with facility and advantage in the investigation of the distribution of intensities in continuous or semi-continuous spectra but difficulties are met with in the case of line spectra.
Paschen 6 has further extended the method and added a number of infra-red lines to the spectra of helium, argon, oxygen and other elements.
In order to record line spectra it is by no means necessary that the receiving instrument (bolometer or radiometer) should be linear in shape, for the separation of adjacent lines may be obtained if the linear receiver be replaced by a narrow slit in a screen placed at the focus of the condensing lens.
It is only recently that owing to the introduction of carbon tubes heated electrically the excitement of the luminous vibrations of molecules by temperature alone has become an effective method for the study of their spectra even in the case of metals.
In the ordinary laboratory the Bunsen flame has become universal, and a number of substances, such as the salts of the alkalis and alkaline earths, show characteristic spectra when suitably placed in it.
The spectra produced under these circumstances have been studied in detail by C. de Watteville.4 Of more frequent use have been electric methods, owing to the greater intensity of the radiations which they yield.
The usual method of obtaining spectra by the discharges from a Ruhmkorff coil or Wimshurst machine needs no description.
For the investigation of the spectra of gases at reduced pressures the so-called Plucker tubes (more generally but incorrectly called Geissler tubes) are in common use.
Goldstein 7 was able to show that an increase in the current density is capable of destroying the well-known spectra of the alkali metals, replacing them by quite a new set of lines.
For our immediate purpose these considerations are of importance inasmuch as they bear on the question how far the spectra emitted by gases are thermal effects only.
We generally observe spectra under conditions in which dissipation of energy takes place, and it is not obvious that we possess a definition of temperature which is strictly applicable to these cases.
Considering the great variety of spectra, which one and the same body may possess, the idea lies near that free electrons may temporarily attach themselves to a molecule or detach 'themselves from it, thereby altering the constitution of the vibrating system.
This is most likely to occur in a discharge through a vacuum tube and it is just there that the greatest variety of spectra is observed.
In other spectra such " harmonic " ratios were also discovered, but their search was abandoned when it was found that their number did not exceed that calculated by the laws of probability on the supposition of a chance distribution.
Balmer's formula received a striking confirmation when it was found to include the ultra-violet lines which were discovered by Sir William Huggins' in the photographic spectra of stars.
The most complete hydrogen spectrum is that measured by Evershed 8 in the flash spectrum observed during a total solar eclipse, and contains thirty-one lines, all of which agree with considerable accuracy with the formula, if the frequency number n is calculated correctly by reducing the wave-length to vacuo.9 It is a characteristic of Balmer's formula that the frequency approaches a definite limit as s is increased, and it was soon discovered that in several other spectra besides hydrogen, series of lines could be found, which gradually come nearer and nearer to each other as they become fainter, and approach a definite limit.
C. Pickering' used in a special case, presently to be referred to, was put into a more general form by Thiele, who, however, assumes N to have the same value for all spectra, and not obtaining sufficient agreement, rejects the formula.
In the spectra of the alkali metals each line of the trunk is a doublet, and we may speak of a twin trunk springing out of the same root.
In the same spectra the lines belonging to the two branches are also doublets.
This form has the advantage that the constants of the equation when applied to the spectra of the alkali metals show marked regularities.
If we compare together the spectra of the alkali metals, we find that the doublets of the branch series separate more and more as the wave-length increases.
The spectra of magnesium, calcium, zinc, cadmium and mercury, give the two branch series, and each series is repeated three times with constant difference of frequency.
All its lines arrange themselves in two families of series, in other words, the spectrum looks like that of the superposition of two spectra similar to those presented by the alkali metals.
The conclusion which was originally drawn from this fact that helium is a mixture of two gases has not been confirmed, as one of the spectra of oxygen is similarly constituted.
We must refer to Kayser and Runge's Handbuch for further details, as well as for information on other spectra such as those of silver, thallium, indium and manganese, in which series lines have been found.
It seems remarkable, however, that we should not have succeeded yet in reproducing in the laboratory the trunk and main branch of the hydrogen spectrum, if the spectra in question really belong to hydrogen.
Such spectra seem to be characteristic of complex molecular structure, as they appear when compounds are raised to incandescence without decomposition, or when we examine the absorption spectra of vapours such as iodine and bromine and other cases where we know that the molecule consists of more than one atom.
The bands often appear in groups, and such spectra containing groups of bands when viewed through small spectroscopes sometimes give the appearance of the flutings of columns.
Hence the name " fluted spectra," which is sometimes applied.
This fact bridges over the distinction between the band and line spectra.
Experimentally we should be confined to a strict investigation of absorption spectra, because in the electric discharge temperature has no definite meaning, and variations of pressure and density are not easily measured.
Halm,' to whom we owe a careful comparison of the above equation with the observed frequencies in a great number of spectra, attached perhaps too much weight to the fact that it is capable of representing both line and band spectra.
A discussion of band spectra on a very broad basis was given by Thiele,' who recommends a formula - q +qi(s+c)+ +qr(s+c)r n in the discharge, except within the region of the kathode glow.
Dufour' discovered that the lines into which the band spectra of the fluorides of the alkaline earths may be resolved widen towards the red under increased pressure.
Leaving the consideration of radical changes of a vibrating system out of account for the present, the minor differences which have been observed in the appearances of spectra under different sparking conditions are probably to a large extent due to differences in the quantities of material examined, though temperature must alter the violence of the impact and there is a possible effect due to a difference in the impact according as the vibrating system collides with an electron or with a body of atomic dimensions.
Their presence indicates the characteristic difference between the spark and the arc. The name is due to Sir Norman Lockyer, who has studied these lines and drawn the attention of astronomers to their importance in interpreting stellar spectra.
It was soon found, however, that compounds possess their own characteristic spectra, and that an element may give under special conditions of luminosity several different spectra.
When we now speak of the identification of spectra we like to include, wherever possible, the identification of the particular compound which is luminous and even - though we have only begun to make any progress in that direction - the differentiation between the molecular or electronic states which yield the different spectra of the same element.
Paschen proved that the emission spectra of water vapour as observed in an oxyhydrogen flame, and of carbon dioxide as observed in a hydrocarbon flame may be obtained by heating aqueous vapour and carbon dioxide respectively to a few hundred degrees above the freezing point.
The spectra experimented on by Paschen were band spectra, but as these split up into fine lines the possibility of homogeneous radiation in pure thermal oscillation may be considered as established.
Pringsheim, who, by a series of experiments of undoubted merit, tried to establish that the emission of the line spectra of the alkali metals was invariably associated with a reduction of the metallic oxide.
Pringsheim seems, however, to have modified his view in so far as he now seems to consider that the spectra in question might be obtained also in other ways, and to attach importance to the process of reduction only in so far as it forms an effective inciter of the particular spectra.
In spite of the fact that C. Fredenhagen has recently attempted to revive Pringsheim's original views in a modified form - substituting oxidation for reduction - we may consider it as generally admitted that the origin of spectra lies with vibrating systems which are definite and not dependent on the method of incitement.
Thus one of the most common spectra is that seen at the base of every candle and in every Bunsen burner.
Compounds generally show spectra of resolvable bands, and if an elementary body shows a spectrum of the same type we are probably justified in assuming it to be due to a complex molecule.
If oxygen is rendered luminous by the electric discharge, a series of spectra may be made to appear.
We have therefore five distinct spectra of oxygen apart from the absorption spectra of ozone.
Stark, who exa*r;ned the spectra of the so-called " canal-rays " (Canalestrahlen).
Stark discovered that in the case of the series spectrum of hydrogen and of other similar spectra the lines were displaced indicating high velocities; in other cases no displacements could be observed.
The conclusion seemed natural that the spectra which showed the Doppler effect were due to vibratory systems which had an excess of positive charge.
Previous to Stark's investigation P. Lenard 2 had concluded that the carriers of certain of the lines of the flame spectra of the alkali metals are positively charged.
The spectra, for instance, of the oxides and haloid salts of the alkaline earths show great resemblance to each other, the bands being similar and similarly placed.
It is in the case of the absorption spectra of liquids that we can most often discover some connexion between vibrations of a complex system and that of the simpler systems which form the complex.
Kundt,' who initiated this line of investigation, came to the conclusion that the absorption spectra of certain organic substances like cyanin and fuchsin were displaced towards the red by the solvent, and that the displacement was the greater the greater the dispersive power of the solvent.
Gladstone, 2 at an early period of spectroscopy, examined the absorption spectra of the solution of salts, each constituent of which was coloured.
Soret has confirmed, for the ultra-violet rays, Dr Gladstone's conclusions with regard to the identity of the absorption spectra of different chromates.
In many of these cases the observed facts might perhaps be explained by dissociation, the undissociated compound producing no marked effect on the spectra.
Huntingdon examined by photographic methods the absorption spectra of a great number of organic compounds.
It will be advantageous if the spectra of ammonia, benzene, aniline and dimethyl aniline be compared, when the re-' markable coincidences will at once become apparent, as also the different weighting of the molecule.
The absorption spectra of cobalt and didymium salts also offer many.
The collimator has a vertical slit at its outer end, the width of which may be regulated by a micrometer screw; in some instruments one half of the slit is covered by a small total reflection prism which permits the examination of two spectra simultaneously.
By suitably replacing the ocular of the observing telescope in an angular vision spectroscope by a photographic camera, it is possible to photograph spectra; such instruments are termed spectrographs.
It was discovered in 1861 by Sir William Crookes, who, during a spectroscopic examination of the flue-dust produced in the roasting of seleniferous pyrites occurring at Tilkerode in the Harz, observed a green line foreign to all then known spectra.
The anthelia (from the Greek duet, opposite, and i Xcos, the sun) are coloured red on the inside, the outside being generally colourless owing to the continued overlapping of many spectra.
Other types of triple object-glasses with reduced secondary spectra have recently been introduced.
If both the bodies are luminous, especially if they do not differ much in brilliancy, the motion of revolution is shown by a periodic doubling of the lines of the spectrum; when one body is moving towards us and the other away their spectral lines are displaced (according to Doppler's principle) in opposite directions, so that all the lines strong enough to appear in both spectra appear double; when the two bodies are in conjunction, and therefore moving transversely, their spectra are merged into one and show nothing unusual.
For purposes of precise scientific investigation the study of spectra is generally more suitable than the vague and unsatisfactory estimates of colour, which differ with different observers.
The spectrum consists of of a continuous band of light crossed by a greater or Spectra .
In 1876 he successfully applied photography to the study of the ultra-violet region of stellar spectra.
Various schemes of classification of spectra have been used.
Most of the above investigators, besides giving a general result, have determined the apex separately for bright and faint stars, for stars of greater or less proper motion, and in some cases for stars of Sirian and Solar spectra.
On considering the distribution of the stars according to their spectra, it appears that the Type II.
The result of course only applies to the brighter stars, for we have very little knowledge of the spectra 'of stars fainter than about magnitude 7.5.
Solar spectra are such spectra.
Solar spectra are effects of those gases.
Barium and its salts can be readily detected by the yellowishgreen colour they give when moistened with hydrochloric acid and heated in the Bunsenflame, or by observation of their spectra, when two characteristic green lines are seen.
Their spectra are only imperfectly known in a few cases, and the bearing of the absorption on the life-history is still a mystery.
But to him belongs the merit of having, most probably without knowing what had already been done, enunciated a complete account of its theory, and of thus having firmly established it as a means by which the chemical constituents of celestial bodies can be discovered through the comparison of their spectra with those of the various elements that exist on this earth.
Other information about the spots is given below, in connexion with their spectra.
The spectrum taken near the limb of the sun shows increased general absorption, but also definite peculiarities of great interest in connexion with the spectra of the spots, which it will be convenient to describe first.
It is virtually identical with a group known as the " yellow stars," of which the most prominent examples are Capella, Pollux and Arcturus; this is not the most numerous group, however; more than one half of all the stars whose spectra are known belong to a simpler type in which the metallic lines are faint or absent, excepting hydrogen and sometimes helium, which declare themselves with increased prominence.
It appears that Newton made the mistake of supposing that all prisms would give a spectrum of exactly the same length; the objections of his opponents led him to measure carefully the lengths of spectra formed by prisms of different angles and of different refractive indices; and it seems strange that he was not led thereby to the discovery of the different dispersive powers of different refractive substances.
The number of stars is so vast that statistical methods can be applied to many of the characters which they exhibit - their spectra, their apparent and absolute luminosity, and their arrangement in space.
To the statistician of the stars, catalogues of spectra, magnitude, position and proper motions are of the same importance that census tables are to the student of humanity.
He observed, in 1823, dark lines in stellar spectra which Kirchhoff's discovery supplied the means of interpreting.
The materials for it were rapidly accumulated by the use of an objective prism, that is, of a prism placed in front of, instead of behind the object-lens, by which means the spectra of all the stars in the field, to the number often of many score, imprinted themselves simultaneously on the sensitive plate.
By comparison with their analogues in the laboratory it can be determined whether, in which direction, and how much, lines of recognized origin are displaced in the spectra of the heavenly bodies.
There are many lines in the spectra of the stars, as well as of the nebulae, which are not certainly identified with those belonging to any elements known to our chemistry.
If a grating is placed as object before the microscope objective, Abbe showed that in the image there is intermittent clear and dark banding only, if at least two consecutive diffraction spectra enter into the objective and contribute towards the image.
If in this case the aperture of the objective be so small, or the diffraction spectra lie so far from each other, that only the pencil parallel to the axis, i.e.
The resemblance is greater the more diffraction spectra enter the objective.
If this object be viewed by the objective, so that at least the diffraction spectra of 1st order pass the finer divisions, then the corresponding diffraction phenomenon in the back focal plane of the objective has the appearance shown in fig.
If one cuts out by a diaphragm in the back focal plane of the objective all diffraction spectra except the principal maximum, one sees in the image a field divided into two halves, which show with different clearness, but no banding.
By choosing a somewhat broader diaphragm, so that the spectra of 1st order can pass the larger division, there arises in the one half of the field of view the image of the larger division, the other half being clear without any such structure.
By using a yet wider diaphragm which admits the spectra of 2nd order of the larger division and also the spectra of 1st order of the fine division, an image is obtained which is similar to the object, i.e.
This can be done by cutting off the chief maximum and using only the diffracted spectra for producing the image.
As was seen when discussing the physical theory, the minute details of the object cause diffractions, and can only be examined if the objective can take up at least two consecutive diffraction spectra.
Students generally use the gas Emission program after observing the spectra emitted by gas discharge tubes.
The velocity dispersion of the cluster galaxies can be measured from optical spectra.
Common minerals from different occurrences have been shown to have distinctive PCL spectra.
It is important to understand the origin of the graphs and curves displaying the excitation and emission spectra for a given fluorochrome.
If it doesn't end up with perfectly horizontal spectra then something has gone wrong.
Infrared spectra show the presence of different types of water molecules and/or hydroxyl ions.
Jacobian spectra for both and find tangent altitude range where these are distinguishable.
These codes also allow some data manipulation, eg adding spectra, multiplying by a constant, interpolation.
Some years back, there began experiments with different IR spectra used to kill harmful microorganisms.
We address these objectives by producing albedo maps, and reflection and emission spectra, and observing stellar occultations.
A database of Raman spectra of inorganic pigments is maintained here.
Their spectra also have the same large redshift of the quasars in the cluster.
Examples include the discovery of pulsars while studying interplanetary scintillation; finding quasars as stars with strange spectra.
In effect, a spectrogram is built up from a multitude of power spectra of short, overlapping time segments of the signal.
Both laser excitation spectra and dispersed fluorescence spectra can be obtained with this arrangement.
Here we use Monte Carlo method to simulate the spectra that may have been measured by the Compton suppression spectrometer.
Comment This contains the width of the mask used to extract the grating spectra from the LE image.
Clearly obtaining spectra for the flanking field 15 m sources is a high priority.
There is also a list of commonly lost fragments, helpful for interpreting spectra.
The associated vibrational spectra provide an opportunity to demonstrate the nature of the transition.
Some years back, there began experiments with different ir spectra used to kill harmful microorganisms.
It includes entries for directional wave spectra, one-dimensional wave spectra and short term statistics.
However, the pattern of lines did not match any of the lines seen in thousands of stellar spectra gathered over a hundred years.
The high signal to noise ratio in alumina fluorescence spectra is not achieved in the true Raman spectra developed by other types of ceramic.
The absorption spectra of pure samples of some of these pigments are shown in the graph on the left.
An average of ten reflectance spectra was collected for each target surface.
The telescopes would combine infrared light to produce high-resolution spectra of the atmospheres of distant planets.
Recent experimental studies of photoelectron spectra provide support for theories.
Cross disperser a low dispersion prism or grating separating the various orders of spectra typically in an echelle spectrograph.
Atomic spectra generally consist of many sharp lines which, in the absence of spectral overlap, appear symmetrical.
Based on these centroids, the relationship between two different local spectra is characterized by an affine transformation.
Vogel of Potsdam introduced the method of photographing stellar and terrestrial spectra on the same plate, and in this way obtained an immense advance in the ease and precision of observation.
To derive the stellar velocity in the line of sight relative to the observer it was then necessary to assume that the normal wave-lengths of the stellar and terrestrial spectra are accurately known.
On this account, observers have until now limited themselves to a partial treatment of such spectra, measuring only a small number of lines, whereby the major part of the rich material present in the plate remains unutilized."
He showed that in the spectra of the fixed stars many of the dark lines were different from those of the solar spectrum, whilst other wellknown solar lines were wanting; and he concluded that it was not by any action of the terrestrial atmosphere upon the light passing through it that the lines were produced.
If the one set of lines exactly bisect the intervals between the others, the grating interval is practically halved, and the previously existing spectra of odd order vanish.
From the regular progression of the lines in the X-ray spectra of different elements Moseley was able tc indicate the number of elements yet to be discovered, and he cleared up certain anomalies in the periodic tables of the elements.
In his optical researches, Optiska Undersiikningar, presented to the Stockholm Academy in 1853, he not only pointed out that the electric spark yields two superposed spectra, one from the metal of the electrode and the other from the gas in which it passes, but deduced from Euler's theory of resonance that an incandescent gas emits luminous rays of the same refrangibility as those which it can absorb.
It has been denied by some that pure thermal motion can ever give rise to line spectra, but that either chemical action or impact of electrons is necessary to excite the regular oscillations which give rise to line spectra.
In that case the main branch is found to represent the new series if a' and b 1 are also put equal to zero, so that n l r I I N = 4 - y2' where r takes successively the values 1.5, 2.5, 3.5 A knowledge of the constants now determines the trunk series, which should be n I I N - (I,5)2 The least refrangible of the lines of this series should have a wavelength 4687.88, and a strong line of this wave-length has indeed been found in the spectra of stars which are made up of bright lines, as also in the spectra of some nebulae.
It is no doubt important to recognize that the two types of spectra seem to represent two extreme cases of one formula, the significant difference being that in the line spectrum the distance between lines diminishes as we recede from the head, while in the case of the band it increases, at any rate to begin with.
The spectrum is of the third type with bright hydrogen emission lines (see below, Spectra of Stars).
Hence above the spots there are vapours of temperature low enough to give the banded spectra of this refractory metal, while only line spectra of sodium, iron and others fusible at more moderate temperatures are found (see also Spect Roheliogra Ph).
Absolute reflectance was calculated by ratioing the collected target and reference spectra.
Chemical shifts in the nuclear magnetic resonance spectra of molecules containing polar groups.
If the input file is 2-D data, then it is treated as a set of 1-D spectra and each is scrunched individually.
If no suitable slew data are available, then method 4 uses standard background spectra.
Infrared spectra of the molecule CH 4 - one of at least ten new molecules detected by ISO.
He showed, from spectra of the galaxies, that there was an increase in the velocity of recession with distance.
Depending on the wavelength range, the telluric lines in the standard spectra can be used.
The similarity of action spectra for thymine dimers in human epidermis and erythema suggests that DNA is the chromophore for erythema.
Recently we have developed methods to computer simulate the infrared and Raman spectra of important zeolite framework types.
Forrester Creations and Spectra Fashions were two rival fashion houses.
Over the years, their rivalry resulted in Forresters working at Spectra and Sally Spectra trying to take over Forrester Creations.
In 2002, Bantam Spectra published a Star Wars character guide to help fans keep up.
The application of photography to exact astronomy has created the necessity for new forms of apparatus to measure the relative positions of stellar and planetary images on photographic plates, and the relative positions of lines in photographic spectra.
He constructed a map of as many as 576 of these lines, the principal of which he denoted by the letters of the alphabet from A to G; and by ascertaining their refractive indices he determined that their relative positions are constant, whether in spectra produced by the direct rays of the sun, or by the reflected light of the moon and planets.
Examination of the absorption spectra of coloured compounds shows that certain groupings displace the absorption bands in one direction, and other groupings in the other.
In different gratings the lengths of the spectra and their distances from the axis were inversely proportional to the grating interval, while with a given grating the distances of the various spectra from the axis were as i, 2, 3, &c. To Fraunhofer we owe the first accurate measurements of wave-lengths, and the method of separating the overlapping spectra by a prism dispersing in the perpendicular direction.
It then appeared that under certain angles of incidence parts of the resulting spectra were completely polarized.