422 Prof. Airy's Remarks on Sir David Brewster's Paper 



You mention, and justly, that the undulatory theory is de- 

 fective as a physical representation of the phenomena of light. 

 I imagine that any theory must be defective in this point. But 

 is the undulatorv theory more or less defective than the mole- 

 cular theory? 1 o assist in forming an answer to this question, 

 I will point out two or three facts. The theory of undula- 

 tions explains well the reflexion at the surfaces of transparent 

 media; which Newton's theory can hardly be strained to ex- 

 plain. With certain assumptions, it gives laws for the inten- 

 sity of the reflected light, which your subsequent experiments 

 on the position of the plane of polarization have confirmed ; no 

 one has even conceived how such a calculation could be com- 

 menced on the molecular theory. It explains the relation 

 (discovered by you) between the polarizing angle and the re- 

 fractive index; an explanation perfectly inconceivable on the 

 molecular theory. It explains with less certainty the elliptic 

 polarization at total internal reflexion, and it does not at all 

 explain the elliptic polarization at metallic reflexion ; but the 

 molecular theory is unable even to give a notion of these kinds 

 of light. Lastly, it explains well the connexion between double 

 refraction and polarization; an explanation which has been 

 hailed by every philosopher who has examined it as the greatest 

 addition made to our physics since the days of Newton ; and 

 one which it will be useless to attempt on a hypothesis of 

 emission. 



The dispersion is doubtless a formidable objection ; though 

 it has been shown that the explanation may be completed by 

 the introduction of causes analogous to those which act in 

 other cases. But is not the dispersion a formidable objection 

 to the molecular theory ? I confess that I have no distinct con- 

 ception of the supposition which must be made in order to 

 explain dispersion on Newton's system. It must be remem- 

 bered that the cause must explain the connexion between the 

 refractive index and the length of the waves or fits; as it is 

 now certain that the smallest change in the latter is accom- 

 panied in every instance by a change in the former. 



I now come to the ostensible subject of the paper, — absorp- 

 tion. I avow, as fully as any opposer of theory can desire, that 

 no explanation of absorption has been given upon the undula- 

 tory system. I assert as fully, that no explanation has been 

 given, or seems likely to be given, on the theory of emission. 

 If we are at present called on to decide between two theories, 

 this subject appears to me to be unimportant. If we are to de- 

 cide whether there shall exist any theory of light at all, the re- 

 solution of the question will depend upon our determining whe- 

 ther absorption must necessarily enter into a theory of light. 



