SICT. XXIV. OBJECTIONS REMOVED. 193 



of the different colors at seven fixed points, determined 

 by seven of the principal dark and bright lines. Profes- 

 sor Powell, availing himself of these measures, has made 

 the requisite computations, and has found that the coin- 

 cidence of theory with observation is perfect for ten 

 substances whose refrangibility had been previously de- 

 termined 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 theoiy; and as there can 

 hardly be a doubt that dispersion hi all other bodies will 

 be found to follow the same law, the undulatory theory 

 of light may now be regarded as completely established. 

 It is however an express condition of the connection be- 

 tween the velocity of light and the length of its undula- 

 tions, that the intervals between the vibrating 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 colors are the same. 



SECTION XXIV. 



Chemical or Photographic Rays of the Solar Spectrum Messrs. Scheele, 

 Ritter, and Wollaston's Discoveries Mr. Wedgewood and Sir Humphry 

 Davy's Photographic Pictures The Calotype The Daguerreotype 

 The Chromatype The Cyanotype Sir John Herschel's Discoveries in 

 the Photographic or Chemical Spectrum Mons. E. Becquerel's Discovery 

 of Inactive Lines in the Chemical Spectrum. 



THE solar spectrum has assumed a totally new char- 

 acter from recent analysis, especially the chemical por- 

 tion, which exercises an energetic action on matter, pro- 

 ducing the most wonderful and mysterious changes on 

 the organized and unorganized creation. 



All bodies are probably affected by light, but it acts 

 with greatest energy on such as are of weak chemical 

 affinity, imparting properties to them which they did 

 13 R 



