192 OBJECTIONS REMOVED. SECT. XXIII. 



come from the sun to the earth with the same velocity. 

 If, indeed, the velocities of the various rays were differ- 

 ent 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 conse- 

 quently 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 be- 

 tween 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 

 discoveiy of M. Fraunhofer furnishes the means of 

 doing so. 



That philosopher obtained a perfectly pure and com- 

 plete colored spectrum with all its dark and bright lines 

 by the interference of light alone, from a sunbeam pass- 

 ing 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 be- 

 tween them are precisely proportional to the differences 

 of these lengths. He measured the lengths of the waves 



