(562.,) 
Chemical 
action of 
light. 
Cuapr. V., § 7.] 
the undulatory theory. That theory, in its simple 
form, enables us indeed to explain clearly enough 
the refraction of light owing to a change of velocity 
in the wave as it passes from one medium to an- 
other; but it assigns no reason why that change of 
velocity should be different for light of various co- 
lours, in other words, of different wave-lengths. The 
corpuscular theory, on the other hand, furnishes at 
least a plausible explanation, by assuming a variable 
attraction between refracting media and the mole- 
cules composing the different rays. 
When, however, undulationists were pressed on 
the subject, it was easy to see the direction in which 
at least a plausible explanation might be sought. In 
the usual form of equation for vibrations in air, 
given by Lagrange, the integration is effected by as- 
suming the intervals between the particles evanescent 
compared with the length of a wave. This is per- 
fectly true in the case of sound, and all sounds ap- 
pear in consequence to travel uniformly. But should 
it fail in the case of light, that is, should the inter- 
vals of the ethereal particles bear some sensible ratio 
to that very small quantity, the length of a wave, 
what would be the result? M. Cauchy has made 
out, by a very complex analysis, that in this case the 
longer waves will travel most rapidly, and conse- 
quently be least refracted. Several other writers, 
especially Professor Kelland, obtained similar re- 
sults; and Mr Airy, by very simple, though only 
approximate, considerations, showed the dependence 
of refraction on the length of a wave. M. Cauchy’s 
memoir appeared in 1835 at Prague, in a bulky and 
abstruse form: the mathematical investigations are 
very long and complex, the numerical verifications 
OPTICS.—M. CAUCHY—RITTER. 
921 
scarcely less so.?_ The indices of refraction, observed 
by Fraunhofer, for different lines of the spectrum, in 
different kinds of glass, together with the correspond- 
ing wave-lengths for these rays, formed the principal 
data for comparison. But others have since been 
obtained by Rudberg and Professor Powell, and care- 
fully compared with M. Cauchy’s theory by the lat- 
ter, and the coincidence appears satisfactory.’ But 
in estimating the value of this coincidence, it is to 
be observed that of 7 indices of refraction observed 
for each substance, 3 must be used to ascertain the 
constants in the formula, and only 4 remain to be 
calculated. 
One difficulty, however, remains. 
light travel through space with variable velocities, the ™ 
consistent with observation. 
difficulty by the following hypothesis respecting ethe- 
real media :—The existence of transversal vibrations 
(according to him) requires that the law of force be- 
tween particle and particle of ether must not be inter- 
mediate between the inverse 2d and inverse 4th power 
of their distance. In the former case the force is an 
attractive, in the second a repulsive one, In the first, 
the velocity of propagation depends on the length of 
a wave, in the second it is independent of it. [The 
first case, too, alone will be consistent with perma- 
nent longitudinal vibrations.| Consequently if we 
suppose that in free space the particles of ether are 
arranged in close order, and exert a repulsive force 
on each other, no dispersion results; but in refract- 
ing media, supposing the distance of the molecules 
increased, and the mutual action attractive, then dis- 
persion occurs.* 
§ 7. Rirrer.—Chemical Rays of the Spectrum.—Niepce; DacuErRE; Mr Tatsor. Art of Helio- 
graphy or Photography—Daguerreotype—Calotype.—Professor STOKES. 
dered visible—F luorescence. 
A very curious chapter of the history of Light re- 
mains to be written, respecting the chemical ener- 
gies which itis capable of exerting, or which at least are 
foundin those parts of thesolar rays which are dispersed 
by a prism. These are in part luminous and partly 
invisible under ordinary circumstances, the latter pos- 
sessing these chemical qualities in a still higher degree 
than the others. Though not perhaps very closely 
associated with the optical discussions of the previous 
sections, it seems impossible to separate this part of 
the subject from the rest, since the rays called Che- 
mical may, as we have reason to think, be reflected, 
refracted, polarized, absorbed, and made to interfere 
like visible light ; and farther, because to the extreme 
Chemical Rays ren- 
limit of their sensible action they may, by certain 
treatment, be made visible to the eye. The art of 
photography, though belonging quite as much to che- 
mistry as optics, being a means of inquiry into the 
qualities of the solar radiations invaluable to the 
natural philosopher, cannot by anymeans be excluded 
from a sketch, however general, of the progress of 
physical science. 
J. W. Rirrer, Professor of Chemistry at Jena, (563.) 
and well-known for his numerous contributions Ritter. 
to the earlier progress of voltaic electricity, has 
the merit of having first clearly pointed out in 1801 
the separate existence of chemical rays in the spec- 
trum which extend beyond the most refrangible or 
1 In all these cases the expression for refrangibility depends on the ratio of the sine of an arc to the are itself; which are 
again includes the ratio of Ax, the distance of the molecules to a, the length of a wave. When Ax becomes very small, the first 
ratio becomes unity. 
2A 
3 Powell on the Undulatory Theory. 1841. 
VOL. I. 
portion of these researches had, however, been printed in Paris (privately, I believe) in 1830. 
* Moigno, Repertoire d'Optique, p. 128. , 
A 
If the rays of _ (561.) 
: ° 2 i f 
images of the stars would present tails of colour in- fight in 
M. Cauchy eludes this free space. 
