Reflexion and Refraction. 131 



things in nature. It is no objection to the hypothesis in 

 question, to say that it increases the difficulty of accounting 

 for refraction ; for, as there is positive evidence in favour of 

 the hypothesis, we ought rather to conclude that the common 

 opinion, which attributes refraction to a change of density in 

 the ether, is altogether erroneous. 



In the next place we may remark, that our first hypothesis,* 

 concerning the direction of vibrations in polarized light, will be 

 useful in testing any proposed theory ; for as it now seems to be 

 certain that the vibrations are parallel to the plane of polariza- 

 tion, and not perpendicular to it, as Fresnel supposed, such a 

 direction of the vibrations ought to be a consequence of the 

 theory which we adopt. 



The third hypothesis, or the principle of the preservation of 

 vis viva, is the most natural that can be imagined, inasmuch as 

 it implies only this, that the incident light is equal to the sum 

 of the reflected and refracted lights. Yet it is probable that 

 even this principle, like the law of vis viva in ordinary mechanics, 

 is a result of simpler laws, and will be shown to be so as soon 

 as the true mechanism of light shall be discovered. 



The fourth hypothesis is a very important one, because the 

 whole theory turns upon it ; and therefore, in the beginning of 

 this Paper, a particular account has been given of the manner in 

 which it was originally suggested. If we wish to give a reason 

 for this hypothesis, we might say that the motion of a particle 

 of ether, at the common surface of two media, ought to be the 

 same, to whichsoever medium the particle is conceived to 

 belong ; and as the incident and reflected vibrations are super- 

 posed in one medium, and the refracted vibrations in the other, 

 we might infer that the resultant of the former vibrations ought 

 to be the same, both in length and direction, as the resultant of 

 the latter. At first sight this reasoning appears sufficiently 

 plausible ; but it will not bear a close examination. For as 

 the argument is general, "it would prove that the principle of 



* This hypothesis properly belongs to the laws of propagation, as it relates only 

 to what passes within a given medium. 



K2 



