130 On tJie Laws of Crystalline 



is certain, indeed, that light is produced by undulations, pro- 

 pagated, with transversal vibrations, through a highly elastic 

 ether; but the constitution of this ether, and the laws of its 

 connexion (if it has any connexion) with the particles of bodies, 

 are utterly unknown. The peculiar mechanism of light is a 

 secret that we have not yet been able to penetrate. As a proof 

 of this, we might observe, that some of the simplest and most 

 familar phenomena have never been explained. Not to mention 

 dispersion, about which so much has been fruitlessly written, 

 we may remark, that the very cause of ordinary refraction, or 

 of the retardation which light undergoes upon entering a trans- 

 parent medium, is not at all understood. Much less can it be 

 said that double refraction has been rigorously explained ; its 

 laws alone have been clearly developed by Fresnel. In short, 

 the whole amount of our knowledge, with regard to the propa- 

 gation of light, is confined to the laws of phenomena : scarcely 

 any approach has been made to a mechanical theory of those 

 laws. And if the case of uninterrupted propagation through 

 a continuous medium presents such difficulties, it would be use- 

 less 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 con- 

 trary course ; by taking those laws for granted, and endeavour- 

 ing 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 



* Those who maintain that the density of the ether is different in different media, 

 ought to consider the following question : What function of the three principal 

 indices of a doubly-refracting crystal represents the density of the ether within 

 the crystal P 



