SECT, xxiii.] OBJECTIONS REMOVED, 223 



length of the waves should be found independently of re- 

 fraction, 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 pris- 

 matic 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 coinci- 

 dence 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 

 whose refrangibility has more recently been ascertained by 

 M. Rudberg. Thus, in the case of seven rays in each of 

 twenty different substances, solid and fluid, the dispersion of 

 light takes place according to the laws of the undulatory 

 theory : and, as there can hardly be a doubt that dispersion 

 in all other bodies will be found to follow the same law, the 

 undulatory theory of light may now be regarded as com- 

 pletely established. It is, however, an express condition of 

 the connexion between the velocity of light and the length 

 of its undulations, that the intervals between the vibrat- 

 ing molecules of the ethereal fluid should bear a sensible 

 relation to the length of an undulation. The coincidence 

 of the computed with the observed refractions shows that 

 this condition is fulfilled within the refracting media ; but 

 the aberration of the fixed stars leads to the inference that 

 it does not hold in the ethereal regions, where the velocities 

 of the rays of all colours are the same. 



