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, ina conversation and 
a letter, expressed himself thus to me: “‘ The perusal of Mr. Mae 
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 @ priort 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 be 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 consequently 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 
Cullagh 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 zot 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 hac 
