" On the Absorption of Specific Rays, #r." 421 



of the analysing plate. It will not explain the rings produced 

 with Iceland spar with circularly polarized light. It is unable 

 even to express the nature of circularly and elliptically po- 

 larized light. It will not explain the phaenomena of quartz. It 

 will not explain the rings of biaxal crystals with plane or circu- 

 larly polarized light, in different positions of the analysing plate. 



The phenomena which I have mentioned have all been cal- 

 culated on the undulatory theory, and they agree perfectly 

 with the deductions from that theory: these calculations more- 

 over are all to be found in print. They are also phenomena 

 which ought to be explained by the theories above mentioned 

 if those theories possess any value. They are, therefore, as 

 appears to me, a fair subject of examination to any person who 

 wishes to decide in the choice of a theory. 



I think it unnecessary to remark further upon your sentence, 

 " Twenty theories may all enjoy the merit of accounting for a 

 certain class of facts, &c." because my opinion is sufficiently 

 expressed above. In whatever degree twenty theories may 

 enjoy this merit in conceivable cases, there are not two that 

 enjoy it in optics. And the ground upon which the supporters 

 of the undulatory theory receive that theory, is, not that it ex- 

 plains phaenomena as well as any other theory, but that it ex- 

 plains phenomena which no other theory can explain. 



To the authority of Newton (supposing authority to deserve 

 the least weight when our collection and variety of facts, and 

 our powers of calculation, have been so immensely increased,) 

 I attribute no importance, for the following reason. I think 

 that Dr. Young has fully made out (Phil. Trans. 1802,) that 

 Newton was a believer in the theory of undulations. In his 

 Optics he most cautiously urges the reader to connect no 

 physical conception with the theory of Fits. Nor are reasons 

 wanting for his adoption of the molecular theory for calcula- 

 tion. It was impossible in Newton's time to make any ex- 

 tended calculation on the undulatory theory ; for even the prin- 

 ciple of the coexistence of small vibrations, which occurs in 

 every part of such calculations, was then unknown. "With all 

 the methods necessary for the calculation of a molecular theory 

 he was, on the contrary, perfectly familiar. 



ception; and if he could extend the hypothesis so as to include polarized 

 light (plane, circular, and elliptical), 1 should have a sincere respect for his 

 inventive powers. But if he seriously applied it to the explanation of 

 phaenomena, which are a necessary consequence of the simple fundamental 

 assumptions of the undulatory theory, I should have little esteem for his 

 judgement as a philosopher. The man who, in order to subtract 2 from 3, 

 should wait for the completion of Mr. Babbugc's engine, would not, in my 

 opinion, be acting more absurdly. 



