216 



at present to reconcile such adoption or such rejection with received 

 opinions respecting the mechanism of light ; exhibiting thus, a 

 kind of intellectual courage, in admiring which I am fortified by 

 the opinion of Sir John Herschel, who lately, in a conversation and 

 a letter, expressed himself thus to me : " The perusal of Mr. Mac 

 Cullagh's paper on the Laws of Reflexion and Polarisation in Crys- 

 tals, has, although cursory, produced a very strong impression on 

 my mind that the theory of light is on the eve of some considerable 

 improvement, and that by abandoning for a while the d priori or 

 deductive path, and searching among phenomena for laws simple 

 in their geometrical enunciation, and of more or less wide applica- 

 bility, without (for a while) much troubling ourselves how far 

 those laws may he in apparent accordance with any precon- 

 ceived notions^ or even with what we are used to consider as 

 general principles in dynamics, it may be possible to unite scat- 

 tered fragments of knowledge into such groups and masses as 

 shall afford glimpses of their fitness to combine into a regular 

 edifice." 



The hypotheses which are the bases of Mr. Mac Cullagh's 

 theory of Crystalline Reflexion and Refraction are the following. 

 He supposes that the form of the wave surface in a doubly- 

 refracting crystal is that which was assigned by Fresnel, and that 

 the vibrations are tangential to this surface, but that they are 

 perpendicular to the ray, and con^QC^&aWj parallel to the plane 

 of polarisation ; whereas Fresnel supposed them to coincide with 

 the projection of the ray upon the wave, and consequently to 

 be perpendicular to the plane of polarisation. Professor Mac 

 CuUagh supposes also, with Fresnel, that the vis viva is pre- 

 served, or in other words, that the reflected and refracted 

 lights are together equal to the incident ; but in applying this 

 principle to investigate the refracted vibrations, he supposes, in 

 opposition to Fresnel, that the density of the ether is not changed 

 in passing from one body to another. And he supposes, finally, 

 that the vibrations in two contiguous transparent media are equi- 

 valent ; or, in other words, that the resultant of the incident and 

 reflected vibrations is the same, both in length and direction, as 

 the resultant of the refracted vibrations ; whereas Fresnel had 



