LIGHT.

411

Ij S ht - the seven colours he enumerates can be distinctly recognised, hut shading so far insensibly into each other that a Part II.

positive limit between them can be nowhere fixed upon. Whether these colours be really compound or not, whether ^

some other mode of analysis may not effect a separation depending on some other fundamental difference between

the rays than that of the degree of their refrangibility, is quite another question, and will be considered more at

large hereafter. At present it may be enough to remark, that all probability, drawn from everyday experience,

is in favour of this idea, and leads us to believe that orange, green, and violet are mixed colours ; and red,

yellow, and blue, original ones; the former we everyday see imitated by mixtures of the latter, but never vice

versa. This doctrine has been accordingly maintained by Mayer, in a curious Tract published among his works.

(See the Catalogue of Optical Writers at the end of this Article.) A very different doctrine has, however, beenadvanced by Dr. Young, ( Lectures on Natural Philosophy , i. 441,) in which he assumes red, green, and violet,as the fundamental colours. The respective merits of these systems will be considered more at large hereafter.

(See Index, Composition of Colours.)

Media, as we have seen, differ very greatly in their refractive power, or in the degree in which prisms of one and the 425.

same refracting angle composed of different substances, deflect the rays of light. This was known to the optical phi- Medialosophers who preceded Newton. This great man, on establishing the general fact, that one and the same medium |j. errefracts differently the differently coloured rays, might naturally have been led to inquire experimentally whether p 0 ^. er .the amount of this difference of action were the same for all media. He appears to have been misled by an acci-dental circumstance in the conduct of an experiment, in which the varieties of media in this respect ought to havestruck him,* and in consequence adopted the mistaken idea of a proportional action of all media on the several homo-geneous rays. Mr. Hall, a gentleman of Worcestershire , was the first to discover Newton’s mistake; and havingascertained the fact, of the different dispersive powers of different kinds of glass, applied his discovery successfullyto the construction of an achromatic telescope. His invention, however, was unaccountably suffered to fall intooblivion, (though it is said that he made several such telescopes, some of which still exist,) and the fact wasre-discovered and re-applied to the same great purpose by Mr. Dollond , a celebrated optician in London , onthe occasion of a discussion raised on the subject by some a priori and paradoxical opinions broached byEuler .

If a prism of flint glass and one of crown, of equal refracting angles, be presented to two rays of white 426.light, as A B C, a be, (fig. 97 ;) S C and sc being the incident rays, C R, C V the red and violet rays refracted Differencesby the flint, and c r, cv those refracted by the crown ; it is observed, first, that the deviation produced in either °f disper-the red or violet ray by the flint glass, is much greater than that produced by the crown ; secondly, that the angle S1 °? e *'

R C V, over which the coloured rays are dispersed by the flint prism, is also much greater than the angle rev,over which they are dispersed by the crown ; and, thirdly, that the angles R C V, rev, or the angles of disper-sion, are not to each other as Newton supposed them to be, in the same ratio with the angles of deviationT C R, ter, but in a much higher ratio; the dispersion of the flint prism being much more than in proportionto the deviation produced by it. And if, instead of taking the angles of the prism equal, the refracting angle of thecrown prism be so increased as to make the deviation of the red ray equal to that produced by the flint prism,the deviation of the violet will fall considerably short of such equality. In consequence of this, if the twoprisms be placed close together, with their edges turned opposite ways, as in fig. 98, so as to oppose each other’s 98.action, the red ray, being equally refracted in opposite directions, will suffer no deviation ; but the violet ray,being more refracted by the flint than by the crown prism, will, on the whole, be bent towards the thicker partof the flint prism, and thus an uncorrected colour will subsist, though the refraction (for one ray, at least) iscorrected. Vice versa, if the dispersion be corrected, that is, if the refracting angle of the crown prism, actingin opposition to the flint, he so further increased as to make the difference of the deviations of the red and violetrays produced by it equal to the difference of their deviations produced by the flint, the deviation produced byit will now he greater than that produced by the flint; and the total deviation, produced by both prisms actingtogether, will now be in favour of the crown.

°By such a combination of two prisms of different media a ray of white light may therefore be turned aside 427.considerably from its course, without being separated into its elementary coloured rays. It is manifest, that (sup- R-fAaction.posing the angles of the prisms small, and that both are placed in their positions of minimum deviation) the Ration ^deviations to produce this effect must be in the inverse ratio of the dispersive powers of the two media; for into colours,supposing p, p' to be the refractive indices of the prisms for extreme red rays, and p -f- S p t f g f for extremeviolet, A and A ' their refracting angles, and D and D' their deviations, we have, generally, in the position ofminimum deviation

A . A + D ,A , A + D

p . sin — = sin -—£—> whence <5 p . sin — = i D . cos

, . A'

p '. sin —-j— = sin

2

A'+ D'

2

S p '. sin — = g £ D'. cos

S *

A'+D'

whence, since the prisms oppose each other,

* He counteracted the refraction of a glass, by a water prism. There ought to have been a residuum of uncorrected colour; but,unluckily, he had mixed sugar of lead with the water to increase its refraction, and the high dispersive power of the salts of lead (of which,of course, he could not have the least suspicion) thus robbed him of one of the greatest discoveries in physical optics.

3 H 2