1)0 HISTORY OF OPTICS. 



transmission described above ?" Several of the other queries imply 

 the same persuasion, of the necessity for the assumption of an ethei 

 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 favor of this 

 being the whole of the mechanism, 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 Newton. That propositions existed in the 

 Princlpia which proceeded on this hypothesis, was, with many of 

 these persons, ground enough for adopting the doctrine ; and it had 

 also the advantage of being more ready of conception, for though the 

 propagation of a wave is not very difficult to conceive, at least by a 

 mathematician, the motion of a particle is still easier. 



On the other hand, the undulation theory was maintained by no less 

 a person than Euler ; and the war between the two opinions was car- 

 ried on with o-reat earnestness. The arguments on one side and on 



o o 



the other soon became trite and familiar, for no person explained any 

 new class of facts by either theory. Thus it was urged by Euler 

 against the system of emission, 18 - that the perpetual emanation of 

 light from the sun must have diminished the mass ; that the stream 

 of matter thus constantly flowing must affect the motions of the 

 planets and comets ; that the rays must disturb each other ; that the 

 passage of light through transparent bodies is, on this system, incon- 

 ceivable : all such arguments were answered by representations of the 

 exceeding minuteness and velocity of the matter of light. On the 

 other hand, there was urged against the theory of waves, the favorite 

 Newtonian argument, that on this theory the light passing through an 

 aperture ought to be diffused, as sound is. It is curious that Euler 

 does not make to this argument the reply which Huyghens had made 

 before. The fact really w r as, that he was not aware of the true ground 

 of the difference of the result in the cases of sound and light ; namely, 

 that any ordinary aperture bears an immense ratio to the length of an 

 undulation of light, but does not bear a very great ratio to the length 

 of an undulation of sound. The demonstrable consequence of this 

 difference is, that light darts through such an orifice in straight rays, 

 while sound is diffused in all directions. Euler, not perceiving this 

 difference, rested his answer mainly upon a circumstance by no means 



12 Fischer, iv. 449. 



