156 Royal Society. 
dia were apparently unconnected with those which govern the pro- 
pagation of light in the same medium. It remained to connect these 
laws as parts of one and the same system, and to trace the hypothe- 
tical principles upon which each theory was based, up to some higher 
mechanical principle. This crowning point of the theory has been 
attained by Professor MacCullagh*. Employing the general pro- 
cesses of analytical mechanics, as laid down by Lagrange+, and li- 
miting the general theorems solely by the conditions that the density 
of the zther is constant, and that the vibrations are transversal, he 
has succeeded in deducing, as parts of one and the same general 
theory, not only the laws of propagation in the same medium, pre- 
viously discovered by Fresnel, but also the laws of reflexion which 
take place at the bounding surface of any two media, already disco- 
vered by himself and M. Newmann. ‘The same theory has likewise 
led to the demonstration of those physical principles, which had been 
assumed in the former paper. It has shown that the vis viva is ne- 
cessarily preserved, in the passage of light from one medium into 
another; that the resultants of the vibrations are the same in the two 
media ; and finally, that the vibrations themselves are parallel to the 
plane of polarization. 
This seems to be the most advanced point to which the physical 
theory of light, in its present form, is capable of being pushed ; and 
it is only by the addition of new physical principles, and further in- 
sight into the constitution of the luminiferous medium, that any ul- 
terior progress can be expected. 
Mr. Fox Tavsor. 
The many important discoveries made by you in Photography, 
discoveries to which I have adverted when addressing the Society, 
on another occasion, discoveries which seem, with those of an ana- 
logous nature made by a Neipsce and a Daguerre, to open to us the 
vista of discoveries still more vast and curious, undoubtedly well en- 
title you to the honour of the Rumford Medal at our hands. Your 
papers, indeed, have been so great an ornament to our volumes, that 
we can never sufficiently express our thanks to you for them. [ 
trust that you will not desert so promising a line of inquiry, and that 
our Transactions may receive from you still greater acquisitions of 
knowledge in the path which is traced by light itself. 
Mr. BowMAn. 
It must be always satisfactory for a President of the Royal Society 
to present to one of your profession a Royal Medal for labours which 
have as their instruments, the assiduous application of the noblest 
faculties of reason—as their immediate purpose, the knowledge of the 
sublime truths contained in the wonderful adaptations of the organs of 
* Proceedings of the Royal Irish Academy for December 1839. The 
complete paper has not yet been published. [Phil. Mag.S.3.vol. xxi. p.228.] 
+ Mr. Green appears to have been the first to apply these methods to the 
dynamics of light, in a paper on the laws of reflexion and refraction at the 
surfaces of uncrystallized media, published in the Cambridge Transactions. 
He has failed, however, in assigning the form of the principal function, and 
has consequently been led to erroneous results. 
t See Phil. Mag. S.3. vol. xix. p. 164. 
