24 SCIENTIFIC THOUGHT. 



which they presented when approaching ponderable or 

 attracting substances. Nothing of this kind seemed 

 imaginable on the undulatory theory, which, reasoning 

 from the analogy of sound, considered light to consist 

 in a rapid to-and-fro motion of the ether in the direc- 

 tion of the rays of light. Sidedness or " laterahty " 

 seemed inconceivable. Eays of light possessing this 

 property would (as Fresnel and Arago showed in 1816) 

 eventually even lose their capabihty of interference, that 

 main property discovered by Young, the principal argu- 

 ment for the vibratory theory. " Every day in that 

 remarkable period — when so many great observers were 

 endeavouring to outstrip each other in the career of 

 discovery — was making known modifications and phe- 

 nomena of polarised light which no existing theory was 

 yet competent to explain. It was polarisation which 

 still continued to cast a dark cloud over the hopes and 

 fortunes of the undulating theory." ^ Thus it was 

 natural that the representatives of the astronomical view 

 of nature, who, headed by Laplace, had given so many 

 real and some apparent explanations of complicated phe- 

 nomena, and to whom the conceptions of the projectile 

 theory of light seemed more promising, should think it 

 time to attack the very stronghold of the \abratory theory, 

 namely, the phenomena of interference, exhibited mainly 

 in diffraction, and, by a minute experimental and mathe- 

 matical analysis, show whether these phenomena could 

 not be brought within the pale of their fundamental con- 

 ceptions. For the discoveries of Young and Fresnel had 

 not shaken them. Accordingly the Paris Academy of 



1 Peacock in ' Life of Young,' p. 383. 



