SECT. XXIII. OBJECTIONS REMOVED. 201 



would be different 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 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 dimin- 

 ished; and, as its refraction, and consequently 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 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 obser- 

 vation, 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 discovery of M. Fraunhofer 

 furnishes the means of doing so. 



That philosopher obtained a perfectly pure and complete 

 coloured spectrum, with all its dark and bright lines, by the 

 interference of light alone, from a sunbeam passing 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 coloured rays depend only on the lengths of 

 their waves, and M. Fraunhofer found that the intervals between 

 them are precisely proportional to the differences of these lengths. 

 He measured the lengths of the waves of the different colours at 

 seven fixed points, determined by seven of the principal dark 

 and bright lines. Professor Powell, availing himself of these 

 measures, has made the requisite computations, and has found 

 that the coincidence of theory with observation is perfect for ten 

 substances whose refrangibility had been previously determined 

 by the direct measurements of M. Fraunhofer, and for ten others 



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