THE ELECTRIC AND LUMINIFEROUS MEDIUM. 
727 
his own dynamical basis, untenable, and leads to the correct result only by accident,— 
and that the credit of the solution of the fundamental dynamical problem of 
Physical Optics belongs essentially to MacCullagh. 
12. To return now to the course of the development of optical doctrine in 
MacCullagh’s hands, he recounts in straightforward fashion,"^ somewhat after tlie 
custom usual with Faraday, the way in wdiich after successive trials he wns at last 
guided to the formal law’s w’hich govern the phenomena of reflexion. To his success 
two main elements contributed ; the bent of his genius led him to apply the methods 
of the ancient Pure Geometry, of which he was one of the great masters, to the ques¬ 
tion, and this resulted in simple conceptions, such as the principle of equivalent 
vibrations already explained, which are applicable to the most geneinl aspect of the 
problem; while the variety and exactness of the experiments of Brewster and 
Seebeck on the polarization of the light reflected from a crystal gave him plenty of 
material by which to mould his geometrical view’s. The sinqde theoremst of the 
'polar plane and of transversals, by which he expressed without symbols in the com¬ 
pass of a single sentence, and in twm different ways, the complete solution of the most 
general problem of crystalline reflexion, contrast wdth the very great complexity of 
the analytical solutions of Neumann and Kirchhoff. Thus at the end of this 
paper he remarks that “ several other questions might be discussed, such as the 
reflexion of common light at the first surface, and the internal reflexion at the 
second surface of a crystal but these must be reserved for a future communica¬ 
tion. It would be easy indeed to wnite dowm the algebraical solutions resulting 
from our theory; but this we are not content to do, because the expressions are 
rather complicated, and when rightly treated will probably contract themselves into 
a simpler form. It is the character of all true theories that the more they are studied 
the more simple they appear to be.” “We are obliged to confess that, wdth the 
exception of the law’ of vis viva, the hypotheses ” on which the solution is founded 
“are nothing more than fortunate conjectures. These conjectures are very probably 
right, since they lead to elegant laws which are fully borne out by experiments ; but 
that is all that we can assert respecting them. We cannot attempt to deduce them 
from first principles ; because, in the theory of light, such iDiinciples are still to be 
sought for. It is certain, indeed, that light is produced by undulations, propagated, 
with transversal vibrations, through a highly elastic aether ; but the constitution of 
this aether, and the laws of its connexion (if it has any connexion) with the particles 
* MacCdllaqh, “ On the Laws of Crystalline Reflexion and Refraction,’’ ‘ Trans. R.I.A.,’ XVIII., 
Jan. 9, 1837. 
t MacCullagh, ‘ Collected Works,’ pp. 97 and 176. 
X It is interesting to observe that, in the notes appended to the paper, MacCullagh has actually 
obtained the geometrical solution of this seemingly most complicated question, by means of a very 
powerful and refined application of the principle of reversibility of the motion, which was afterwards 
employed to such good purpose by Sir S. Gr. Stokes, 
