LUMINOUS PROPERTIES OF SPECTRUM.

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refractive indices of water, oil of turpentine, flint, and crownglass, for the rays в to н inclusive (479) :

Water 1st observation. 1.330935 1.331712 1.333577 1.335851 1.337818 1.341293 1-344177

Water 2d

39

1.330977 1.331709 1.333577 1.335849 1-337788 1.341261 1.344162

Oil of Turpentine . 1-470496 1.471530 1.474434 1.478353 1.481736 1.488198 1.493874 Crown-glass 1st specn. 1.524312 1.525299 1.527982 1.531372 1-534337 1.539908 1.554684 Flint-glass 1st specn. 1.602042 1.603300 1.608494 1.614532 1.620042 1.630772 1.640373

482. The intensity of light in the solar spectrum appears to be greatest in the yellow band, and from that space it decreases to both extremities of the whole series of tints. Fraunhofer has exhibited these variations in the light of the different parts of the spectrum by a curve RKL, the ordinates of which indicate the intensity of light in the different parts of the spectrum RV, in which Fraunhofer's lines (479) have

been marked. Taking the ordinate км falling nearly in the boundary between the yellow and orange as unity, the following will represent the illuminating power of the spectrum in the different portions occupied by Fraunhofer's rays; the red extremity being indicated by R, and the violet by v :

483. The calorific powers of the spectrum increase from the violet to the red extremity, and even extend beyond it, the obscure space bounding the red extremity possessing a higher temperature than the red band itself (H 467); so that it is evident, that when luminous undulations are propagated through a prism, a certain amount of them move with too little rapidity to communicate to the eye the sensations of light, and are only to be recognized by their calorific effects. These rays of non-luminous heat are less refrangible than the rays of red light, and are therefore found in the greatest abundance beyond the band of that colour.

These calorific rays have their situation altered according to the refracting medium of which the prism is constructed; being in the greatest number in the yellow band, when a prism of water; in the orange, when one of sulphuric acid; in the middle of the red, when one of crown glass; and beyond the red, when a prism of flint glass is used.

From the observations of Nobili and Melloni, on a spectrum produced by a rock salt prism, the highest temperature was found beyond the red, and about as far distant from it on one side as the blue band was from it on the other. lowing were the results obtained by Sir H. Englefield:

The fol

Temp. by Fahrenheit's thermometer 56° 58° 62° 72° 79°

484. The chemical action of solar light, in producing chemical combination and decomposition, has been long known, and this like the heating power appears to reside in greater intensity at one end of the spectrum than the other. This may be shown by dipping in a solution of nitrate of silver a slip of paper, previously washed over with a solution of common salt; on drying this, and exposing it to the action

CALORIFIC AND CHEMICAL PROPERTIES OF THE SPECTRUM. 333

of the solar spectrum (147), a very remarkable effect will be observed. In the course of a few minutes the chloride of silver with which the paper was imbued, becomes of a deep slate colour in the violet, and in the sombre space beyond it; whilst in the yellow, orange, and red, it remains unaffected, its colour being less altered in the blue than in the violet, and scarcely at all changed in the green. Thus the chemical action of the different rays of the spectrum appears to be concentrated in the violet band, and in the dark space beyond it, at the directly opposite end to the seat of the calorific rays. So that there is reason to believe that those undulations which are propagated through a prism (467) with too great rapidity to act on the organ of vision, possess the power of exerting certain chemical effects on many substances, in the same manner that calorific effects are exerted by those undulations which move with too little rapidity to produce the sensation of light. Granting this, we meet with another circumstance in which the propagation of light by the undulation of ether, and of sound by those of air, correspond. For it has been already shown (195), that aerial waves moving with a velocity sufficient to strike the ear less than 32, or more than 1500 times in a second, are inaudible; whilst ethereal undulations, if less frequently repeated than 458 millions of millions, or more frequently than 727 millions of millions of times in a second, are incapable of acting on the visual

organs.

485. If De represent the solar spectrum produced by flint glass, and AB DE the non-luminous portions beyond it,

But

at each extremity, the curve EHB will give an idea of the position of the calorific rays, ACD and the position of the chemical rays. The longest ordinate of the curve EHB falls without the red ray R in the obscure space beyond it, where the calorific effects are most manifest; and the longest ordinate of the chemical curve ACD falls in the dark space beyond the violet ray v, where the action on chloride of silver appears to be most intense. Both curves rise abruptly, and gradually decline to zero at opposite ends of the spectrum. 486. When two or more undulations emanating from the same sources, act on a particle of ether, it oscillates with an intensity corresponding to the combined force of the undulations (422); the same thing occurs, providing the latter are of equal length, or differ by a given number of entire undulations, even when they emanate from different sources, if the waves acting on a particle of ether differ by any fractional number of undulations, they interfere and oppose each other's action, and thus actually produce partial or total darkness. To render this more clear, if any number of rays produced by 1, 2, 3, or any whole number of undulations, act in concert, increase of effect is produced; but when a certain portion of the rays are produced by 1, 2, 3, or any whole number, whilst another set are produced by 11, 21, 31, or any fractional number of undulations, interference, and obscuration takes place, from the mutual checking of a certain number of waves. This is rendered obvious by drawing two sets of waves containing the same number of undulations as AB; any particle of ether at c must be made to assume a movement corresponding to the combined action of A and B, and a corresponding intensity of light will result. Then alter the relative position of AB, so that a may begin one half an indulation later than B, as at A'B', then it is at once seen that they will be always in opposite phases;

B

E

INTERFERENCE OF LIGHT.

335

for any particle of ether at c will be acted on in opposite directions by A'B'; for whilst the undulation r is moving from right to left, E is moving in an opposite direction, and mutually opposing, they will cease to act on a particle of ether at c, producing darkness by the conflict of two luminous undulations. If the waves of light, instead of meeting at the end of an entire half-undulation, encounter at any fractional part of one, partial interference will ensue, and colours will be developed, bearing a relation to the length and velocity of the undulation remaining undestroyed.

In this explanation of the interference of luminous waves of ether, it must be borne in mind that the series of progressive undulations here figured are assumed merely for the sake of facility of demonstration, as it has been already pointed out (422) that undulations of highly elastic media, as ether, consist of a series of alternate expansions and contractions in opposite directions of spherical molecules, and not of any truly progressive movements.

487. We have already seen that the interference of sonorous undulations produce silence (188); and in the extension of this fact to luminous waves, we meet with a striking analogy between the oscillations of particles of ether and of air, the difference being rather in degree than in kind. The combined or diminished action of luminous undulations, bear a remarkable relation to musical discords and concords, the former being produced when sonorous vibrations, differing in their rapidity by fractional portions, interfere; and the latter when similar vibrations, bearing to each other a relation in whole numbers, strike the ear together (197).

488. An experimental demonstration of the interference of luminous undulations may be obtained, by placing on a smooth table two pieces of plate glass cut from the same piece, with their divided faces in contact. Gently incline one towards the other by placing a piece of paper under its edge, and allow a ray of homogeneous light (467) to fall upon