EPOCH OF YOUNG AND FRESNEL. 431 



which we have spoken a little while ago. And the 

 great authority of the period, Newton, adopted the 

 opposite hypothesis, that of emission, and gave it a 

 currency among his followers which kept down the 

 sounder theory for above a century. 



Newton's first disposition appears to have been 

 by no means averse to the assumption of an ether 

 as the vehicle of luminiferous undulations. When 

 Hooke brought against his prismatic analysis of 

 light some objections, founded on his own hypo- 

 thetical notions, Newton, in his reply, said*, "The 

 hypothesis has a much greater affinity with his 

 own hypothesis than he seems to be aware of; the 

 vibrations of the ether being as useful and neces- 

 sary in this as in his." This was in 1672; and we 

 might produce, from Newton's writings, passages of 

 the same kind, of a much later date. Indeed it 

 would seem that, to the last, Newton considered 

 the assumption of an ether as highly probable, and 

 its vibrations as important parts of the phenomena 

 of light ; but he also introduced into his system 

 the hypothesis of emission, and having followed 

 this hypothesis into mathematical detail, while he 

 has left all that concerns the ether in the form of 

 queries and conjectures, the emission theory has 

 naturally been treated as the leading part of his 

 optical doctrines. 



The principal propositions of the Principia 

 which bear upon the question of optical theory are 



8 Phil. Trans, vii. 6087. 



