40 REPORT ON PHYSICAL OPTICS. 



Newton thought that the different refrangibility of the rays of 

 light could be explained by supposing simply that they were bodies 

 of different sizes, the red being greatest and the violet least. It is 

 obvious, however, that this supposition can have no reference to 

 the simple projectile hypothesis held by his followers, or to the 

 demonstration of the law of refraction given in the Principia. It 

 is connected with that more complex theory, in which the mole- 

 cules of light are supposed to excite the vibrations of the ether in 

 the bodies which they meet. 



M. de Courtivron and Mr. Melville proposed to account for the 

 dispersion of light by a difference in the initial velocity of the 

 molecules, the red being swiftest and the violet slowest. But 

 were such the cause of the phenomenon, the dispersion should be 

 proportionate to the mean refraction. Indeed, the hypothesis was 

 abandoned almost as soon as proposed. Its authors had foreseen the 

 consequence that, in the eclipses of Jupiter's satellites, the colour of 

 the light should vary just before immersion, and after emersion ; and 

 the existence of such an effect, in the degree indicated by theory,* 

 was completely disproved by the observations of Mr. Short, t 

 Another consequence of such a difference in the initial velocities of 

 the light of different colours is, that the aberration of the fixed 

 stars should also vary with the nature of the light, and each star 

 appear as a coloured spectrum, whose length is parallel to the 

 direction of the earth's motion. 



According to the modern advocates of the theory of emission, 

 the molecules of light are heterogeneous ; and the attractions 

 exerted on them by bodies vary with their nature, and are, in 

 this respect, analogous to chemical affinities. This supposition, 

 however, as Dr. Young has justly observed, is but veiling our 

 inability to assign a mechanical cause for the phenomenon. 



It is remarkable that Newton himself was the first to suggest 

 that part of the wave-theory, in which the colour of the light is 

 supposed to be determined by the frequency of the ethereal vibra- 



* The duration of this change, according to Mr. Melville, should amount to thirty- 

 two seconds, the velocity of the light of different colours being inversely as their re- 

 fractive indices. (**. Trans., 1753.) This principle, however, as M. Clairaut has 

 shown (Phil. ZYww.,1764), is obviously incorrect. It will easily appear that the initial 

 velocities must vary inversely as the quantity v/M^ 7 !, in order to account for dis- 

 persion ; and that the duration of the expected phenomenon must be even greater than 

 that assigned by Mr. MelviUe. 



t PhU. Trans., 1753. 



