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has revolved through 90°; because, in such positions of the spar as produce the colored images, only two rays are visible at a time, the other two being reflected. When the analysis is accomplished by reflection, if two rays are transmitted by the mica, they are polarized in planes at right angles to each other. And if the plane of reflection of either of these rays be at right angles to the plane of polarization, only one of them will be reflected, and therefore no interference can take place; but in all other positions of the analyzing plate both rays will be reflected in the same plane, and consequently will produce colored rings by their interference.

It is evident that a great deal of the light we see must be polarized, since most bodies which have the power of reflecting or refracting light also have the power of polarizing it. The blue light of the sky is completely polarized at an angle of 74° from the sun in a plane passing through his center.

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A constellation of talent almost unrivaled at any period in the history of science, has contributed to the theory of polarization, though the original discovery of that property of light was accidental, and arose from an occurrence which like thousands of others would have passed unnoticed, had it not happened to one of those rare minds capable of drawing the most important inferences from circumstances apparently trifling. 1808, while M. Malus was accidently viewing with a doubly-refracting prism a brilliant sunset reflected from the windows of the Luxembourg palace in Paris, on turning the prism slowly round, he was surprised to see a very great difference in the intensity of the two images, the most refracted alternately changing from brightness to obscurity at each quadrant of revolution. A phenomenon so unlooked for induced him to investigate its cause, whence sprung one of the most elegant and refined branches of physical optics.

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SECTION XXIII.

Objections to the Undulatory Theory, from a Difference in the Action of Sound and Light under the same circumstances, removed-The Dispersion of Light according to the Undulatory Theory.

THE numerous phenomena of periodical colors arising from the interference of light, which do not admit of satisfactory explanation on any other principle than the undulatory theory, are the strongest arguments in favor of that hypothesis; and even cases which at one time seemed unfavorable to that doctrine have proved upon investigation to proceed from it alone. Such is the erroneous objection which has been made, in consequence of a difference in the mode of action of light and sound, under the same circumstances, in one particular instance. When a ray of light from a luminous point, and a diverging sound, are both transmitted through a very small hole into a dark room, the light goes straight forward and illuminates a small spot on the opposite wall, leaving the rest in darkness; whereas the sound on entering diverges in all directions, and is heard in every part of the room. These phenomena, however, instead of being at variance with the undulatory theory, are direct consequences of it, arising from the very great difference between the magnitude of the undulations of sound and those of light. The undulations of light are incomparably less than the minute aperture, while those of sound are much greater. Therefore when light diverging from a luminous point enters the hole, the rays round its edges are oblique, and consequently of different lengths, while those in the center are direct, and nearly or altogether of the same lengths. So that the small undulations between the center and the edges are in different phases, that is, in different states of undulation. Therefore the greater number of them interfere, and by destroying one another produce darkness all around the edges of the aperture; whereas the central rays having the same phases, combine, and produce a spot of bright light on a wall or screen directly opposite the hole. The waves of air producing sound, on the

contrary, being very large compared with the hole, do not sensibly diverge in passing through it, and are therefore all so nearly of the same length, and consequently in the same phase, or state of undulation, that none of them interfere sufficiently to destroy one another. Hence all the particles of air in the room are set into a state of vibration, so that the intensity of the sound is very nearly everywhere the same. Strong as the preceding cases may be, the following experiment made by M. Arago about twenty years ago seems to be decisive in favor of the undulatory doctrine. Suppose a planoconvex lens of very great radius to be placed upon a plate of very highly polished metal. When a ray of polarized light falls upon this apparatus at a very great angle of incidence, Newton's rings are seen at the point of contact. But as the polarizing angle of glass differs from that of metal, when the light falls on the lens at the polarizing angle of glass, the black spot and the system of rings vanish. For although light in abundance continues to be reflected from the surface of the metal, not a ray is reflected from the surface of the glass that is in contact with it, consequently no interference can take place; which proves, beyond a doubt, that Newton's rings result from the interference of the light reflected from both the surfaces apparently in contact (N. 194).

Notwithstanding the successful adaptation of the undulatory system to phenomena, the dispersion of light for a long time offered a formidable objection to that theory, which has only been removed during the present year by Professor Powell of Oxford.

A sunbeam falling on a prism, instead of being refracted to a single point of white light, is separated into its component colors, which are dispersed or scattered. unequally over a considerable space, of which the portion occupied by the red rays is the least, and that over which the violet rays are dispersed is the greatest. Thus the rays of the colored spectrum whose waves are of different lengths, have different degrees of refrangibility, and consequently move with different velocities, either in the medium which conveys the light from the sun, or in the refracting medium, or in both; whereas rays of all colors

come from the sun to the earth with the same velocity. If, indeed, the velocities of the various rays were different in space, the aberration of the fixed stars, which is inversely as the velocity, would be different for different colors, and every star would appear as a spectrum whose length would be parallel to the direction of the earth's motion, which is not found to agree with observation. Besides, there is no such difference in the velocities of the long and short waves of air in the analogous case of sound, since notes of the lowest and highest pitch are heard in the order in which they are struck. In fact, when the sunbeam passes from air into the prism its velocity is diminished; and as its refraction and consequently its dispersion depend solely upon the diminished velocity of the transmission of its waves, they ought to be the same for waves of all lengths, unless a connection exists between the length of a wave, and the velocity with which it is propagated. Now this connection between the length of a wave of any color and its velocity or refrangibility in a given medium, has been deduced by Professor Powell from M. Cauchy's investigations of the properties of light on a peculiar modification of the undulatory hypothesis. Hence the refrangibility of the various colored rays computed from this relation for any given medium, when compared with their refrangibility in the same medium determined by actual observation, will show whether the dispersion of light comes under the laws of that theory. But in order to accomplish this, it is clear that the length of the waves should be found independently of refraction, and a very beautiful discovery of M. Fraunhofer furnishes the means of doing so.

That philosopher obtained a perfectly pure and complete colored spectrum with all its dark and bright lines by the interference of light alone, from a sunbeam passing through a series of fine parallel wires covering the object glass of a telescope. In this spectrum, formed independently of prismatic refraction, the positions of the colored rays depend only on the lengths of their waves, and M. Fraunhofer found that the intervals between them are precisely proportional to the differences of these lengths. He measured the lengths of the waves

of the different colors at seven fixed points, determined by seven of the principal dark and bright lines. Professor Powell, availing himself of these measures, has made the requisite computations, and has found that the coincidence of theory with observation is perfect for ten substances whose refrangibility had been previously determined by the direct measurements of M. Fraunhofer, and for ten others whose refrangibility has more recently been ascertained by M. Rudberg. Thus, in the case of seven rays in each of twenty different substances solid and fluid, the dispersion of light takes place according to the laws of the undulatory theory; and as there can hardly be a doubt that dispersion in all other bodies will be found to follow the same law, the undulatory theory of light may now be regarded as completely established. It is however an express condition of the connection between the velocity of light and the length of its undulations, that the intervals between the vibrating molecules of the ethereal fluid should bear a sensible relation to the length of an undulation. The coincidence of the computed with the observed refractions shows that this condition is fulfilled within the refracting media; but the aberration of the fixed stars leads to the inference that it does not hold in the ethereal regions, where the velocities of the rays of all colors are the same.

SECTION XXIV.

Chemical or Photographic Rays of the Solar Spectrum-Messrs. Scheele, Ritter, and Wollaston's Discoveries-Mr. Wedgewood and Sir Humphry Davy's Photographic Pictures-The Calotype-The DaguerreotypeThe Chromatype-The Cyanotype-Sir John Herschel's Discoveries in the Photographic or Chemical Spectrum-Mons. E. Becquerel's Discovery of Inactive Lines in the Chemical Spectrum.

THE solar spectrum has assumed a totally new character from recent analysis, especially the chemical portion, which exercises an energetic action on matter, producing the most wonderful and mysterious changes on the organized and unorganized creation.

All bodies are probably affected by light, but it acts with greatest energy on such as are of weak chemical affinity, imparting properties to them which they did

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