REPORT ON OPTICS. 
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production of the rings ; but, as Dr. Young and others have 
allowed that the doctrine of interference is reconcileable with 
the doctrine of emission, the disappearance of the rings is not 
necessarily inexplicable on any theory of emission. 
I regret that 1 am unable to give any satisfactory account of 
the very important optical discoveries of M. Cauchy, and I am 
not aware, indeed, that he has published any detailed account 
of his researches. In one of the Memoirs which he has lately 
printed, among those of the Academy of Sciences, he refers to 
three important results, which he has obtained from the undu- 
latory theory: 
1. The deduction of the law of the tangents which connects 
the polarizing angle with the refractive power of the body. 
2. The explanation of the phenomena of dispersion. 
3. The existence of a triple refraction. 
The inability of the undulatory theory to explain the pheno¬ 
mena of inequal refrangibility, is almost the only exception to 
its universal application in accounting for the most complicated 
phenomena of light. Various attempts, though not very success¬ 
ful ones, have been made to remove this difficulty. Dr. Young 
supposes that the material particles of transparent bodies are 
susceptible of permanent vibrations, somewhat slower than the 
undulations which produce them, and that the velocity of the 
original undulation will be diminished in proportion to their fre¬ 
quency. The Rev. Mr. Challis, adopting Dr. Young’s idea, 
has endeavoured to explain the manner in which the undula¬ 
tions of the aether within bodies are modified by their material 
atoms. He supposes that a sensible reflexion takes place at 
every interruption of continuity in the medium; and he infers 
that the mean effect produced by a retarding cause proportional 
to the reflective power of the atoms, will be to make the con¬ 
densation corresponding to a given velocity, greater in a certain 
proportion than in free space, and to diminish the velocity of 
propagation in the same proportion. Mr. Airy has more re¬ 
cently endeavoured to remove this difficulty, by supposing that 
in refracting media there may be something depending on time, 
which alters their elasticity, in the same manner as in air the 
elasticity is greater with a quick than with a slow vibration of 
particles. 
An anonymous writer, in a very recent Number of The Annals 
of Philosophy , has proposed another hypothesis for obtaining a 
difference of elasticity. He supposes that the aether accumu¬ 
lates itself round the particles of transparent media, and forms 
spheres of a density increasing towards their centres ; and he 
infers that a succession of vibrations communicated through a 
