434 HISTORY OF OPTICS. 



not upon new modifications, but upon the original 

 and unchangeable properties of the rays." (Ques- 

 tion twenty-seven.) 



But yet, even in this state of his views, he was 

 very far from abandoning the machinery of vibra- 

 tions altogether. He is disposed to use such ma- 

 chinery to produce his " fits of easy transmission." 

 In his seventeenth Query, he says 11 , "when a ray 

 of light falls upon the surface of any pellucid body, 

 and is there refracted or reflected ; may not waves 

 of vibrations or tremors be thereby excited in the 

 refracting or reflecting medium at the point of inci- 

 dence ? . . . . and do not these vibrations overtake 

 the rays of light, and by overtaking them suc- 

 cessively, do they not put them into the fits of easy 

 reflection and easy transmission described above ?" 

 Several of the other queries imply the same per- 

 suasion, of the necessity for the assumption of an 

 ether and its vibrations. And it might have been 

 asked, whether any good reason could be given 

 for the hypothesis of an ether as a part of the 

 mechanism of light, which would not be equally 

 valid in favour of this being the whole of the me- 

 chanism, especially if it could be shown that nothing 

 more was wanted to produce the results. 



The emission theory was, however, embraced in 

 the most strenuous manner by the disciples of New- 

 ton. That propositions existed in the Principia 

 which proceeded on this hypothesis, was, with many 

 " Opticks, p. 322. 



