Crystalline Reflexion and Refraction. 69 



case of uninterrupted propagation through a continuous medium presents such 

 difficulties, it would be useless to think of accounting for the laws which subsist 

 at the confines of two media, where the continuity is broken. 



But perhaps something might be done by pursuing a contrary course ; by 

 taking those laws for granted, and endeavouring to proceed upwards from them 

 to higher principles. In this point of view, our second law, or hypothesis, is 

 extremely remarkable ; for it seems to be opposed, in some degree, to the notion 

 that the ethereal molecules are strongly attracted or repelled by the particles of 

 bodies. However that may be, it would appear that a true theory must be in 

 accordance with this hypothesis ; and that any mechanical ideas, which would 

 make the mean density of the ether vary from one medium to another, cannot be 

 admitted to represent the real state of 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,f concerning the 

 direction of vibrations in polarised 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 polarisation, 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 mecha- 

 nism of light shall be discovered. 



The fourth hypothesis is a very important one, because the whole theory turns 



* Those who maintain that the density of the ether is different in different media, ought to con- 

 sider the following question : — What function of the three principal indices of a iloubly refracting 

 crystal, represents the density of the ether within the crystal ? 



t This hypothesis properly belongs tO/the laws of propagation, as it relates only to what passes 

 within a given medium. 



