222 OBJECTIONS REMOVED. [SKCT. xxm. 



to a single point of white light, is separated into its com- 

 ponent colours, 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 coloured 

 spectrum, whose waves are of different lengths, have different 

 degrees of refrangibility, and consequently move with dif- 

 ferent velocities, either in the medium which conveys the 

 light from the sun, or in the refracting medium, or in both ; 

 whereas rays of all colours 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 dif- 

 ferent for different colours, 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 observa- 

 tion. 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 dimin- 

 ished ; and, as its refraction, and consequently its dispersion, 

 depend solely upon the diminished velocity of the transmis- 

 sion of its waves, they ought to be the same for waves of all 

 lengths, unless a connexion exists between the length of a 

 wave and the velocity with which it is propagated. Now, 

 this connexion between the length of a wave of any colour, 

 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 coloured 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 



