60 HISTORY OF OPTICS. 



absorbing media, I cannot consider that lie lias established his point 

 as an exception to Newton's doctrine. In the first place, the analysis 

 of light into three colors appears to be quite arbitral 1 }*, granting all his 

 experimental facts. I do not see why, using other media, he might not 

 just as well have obtained other elementary colors. In the next place, 

 this cannot be called an analysis in the same sense as Newton's analy- 

 sis, except the relation between the two is shown. Is it meant that 

 Newton's experiments prove nothing ? Or is Newton's ' conclusion 

 allowed to be true of light which has not been analysed by absorption ? 

 And where are we to find such light, since the atmosphere absorbs ? 

 But, I must add, in the third place, that with a very sincere admiration 

 of Sir D. Brewster's skill as an experimenter, I think his experiment 

 requires, not only limitation, but confirmation by other experimenters. 

 Mr. Airy repeated the experiments with about thirty different absorb- 

 ing substances, and could not satisfy himself that yi an y case they 

 changed the color of a ray of given refractive power. These experi- 

 ments were described by him at a meeting of the Cambridge Philoso- 

 phical Society.] 



We now proceed to the corrections which the next generation intro- 

 duced into the details of this doctrine. 



CHAPTER IV. 



DISCOVERY OF ACHROMATISM. 



fTUIE discovery that the laws of refractive dispersion of different sub- 

 *- stances were such as to allow of combinations which neutralized 

 the dispersion without neutralizing the refraction, is one which has 

 hitherto been of more value to art than to science. The property has 

 no definite bearing, which has yet been satisfactorily explained, upon 

 the theory of light ; but it is of the greatest importance in its applica- 

 tion to the construction of telescopes ; and it excited the more notice, 

 in consequence of the prejudices and difficulties which for a time 

 retarded the discovery. 



Newton conceived that he had proved by experiment, 1 that light 



1 Opllcks, B. i. p. ii. Prop. 3. 



