214 
we hope) with the rights, or even with the convenience of authors ; 
I think myself allowed to enter more at large into the merits of the 
award, and to lay before you some of the thoughts which the pe- 
rusal of the present prize essay has suggested to my own mind. 
When ordinary light is reflected at the common boundary of 
two transparent and uncrystallized media, as when we see (for ex- 
ample) the reflexion of the sun in water, the reflected light differs 
from the incident in both direction and intensity, according to laws 
which were known to Euclid in so far as they regard direction, 
but of which the discovery, in so far as iztensity is concerned, was 
reserved for the sagacity of Fresnel. In general, the laws which 
regulate the changes of the direction of light have been found easier 
of discovery than those which regulate its changes of intensity; the 
laws of the reflexions and refractions of the lines along which light 
is propagated, than the laws of the accompanying determinations or 
alterations of its planes of polarisation ; or, to express the same dis- 
tinction in the language of the theory of undulations, it has been 
found easier to assign the form of the waves which spread from any 
origin of disturbance through any given portion of the elastic lumi- 
niferous ether, than to assign the directions and relative magnitudes 
of the vibrations which constitute those waves, and the laws which 
regulate the changes of such vibrations, in the passage from one 
medium to another. 
The laws which regulate such changes of vibration, produced 
by reflexion and refraction, at the boundaries of crystallized 
media, have been the special object of Mr. Mac Cullagh’s investi- 
gations, in the paper now beforeus. But in investigating them, he 
has been obliged to consider also the laws which regulate the vi- 
brations of the ether, in the ixterior of a crystallized body, and 
not at its sazface only ; the laws of the propagation as well as 
those of the reflexion and refraction of light. His researches are 
therefore connected intimately with a wide range of optical pheno-: 
mena} and the hypotheses on which his formulz are founded, and 
which seem to have their own correctness proved by the experi- 
ments of many kinds with which they have been successfully com- 
pared, though liable, of course, like every physical induction, to be 
modified in some degree by future observation, appear to be en- 
