(576.) 
924 
liarly so. The action of light appears to be to reduce 
the metallic silver, at least partially, an operation 
which is completed in the daguerreotype process ap- 
parently by the affinity of the mercurial vapour for 
iodine and for oxygen." 
If anything were wanting to show the impossibility 
Importance of separating the scientific arts from the history of 
of the pho- 
tographic 
art. 
science itself, it would be the case before us. The 
art of photography is far in advance of the theory, 
yet it constitutes in itself the greater part of what 
we know in a highly interesting branch of Natural 
Philosophy—the chemical agency of the spectrum. 
The surfaces of Talbot and Daguerre are the philo- 
sophical tools by which farther discoveries can alone 
be made, 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss. VI. 
to form a pure and highly dispersed spectrum by 
passing through two or three prisms in succession, 
associated with a lens. The spectrum is made to 
fall on a glass vessel containing a solution of sul- 
phate of quinine (a colourless fluid), Whilst the red, 
orange, and indeed the greater part of the luminous 
spectrum, pass through as if the fluid had been merely 
water, from about the middle of the violet ‘ the 
path of the rays is marked within the fluid by a sky- 
blue light, which emanates in all directions from the 
fluid, as if it (the medium) had been self-luminous.” 
This appearance extends far beyond the visible violet, 
the presence of the invisible rays (called chemical) 
becoming disclosed by the reaction of the quinine ; 
and the dark bands, both of the visible and the 
(577.) So intimately connected with the discovery of the (usually) invisible spectrum, are marked by obscure 
Chemical Chemical Rays of the spectrum, is that of revealing planes traversing the mass of diffuse light. When 
lod vi. their existence (at least the existence of rays of the the light thus emanating from the interior of the 
red vi- - : ° we ° : ° OF 
sible. spectrum equally refrangible with those possessing quinine is examined by a prism, it is found to con- 
chemical properties) to the sense of sight, that we may 
conveniently include in this section a very brief no- 
tice of Professor Stokes’ remarkable experiments. 
sist of rays of various colours and refrangibilities 
corresponding to those of the ordinary spectrum ; 
from which Mr Stokes concludes that, since only vio- 
(578.) Mr Grorce Gasrizt Stoxes, a fellow of Pembroke let rays and the invisible rays beyond them could have 
Serena College, Cambridge, and senior wrangler in 1841, furnished or excited this emanation, the rays of light 
at present holds the distinguished position of Luca- 
sian Professor of Mathematics. It is almost need- 
less, therefore, to state that his mathematical talents 
are generally acknowledged, and that he has dis- 
played them by a ready application to many difficult 
problems in optics and mechanics which had not pre- 
viously been accomplished, I may refer, however, to 
the integration of complex differential equations oc- 
curring in the theory of the flexure of solids (see art. 
364), and in that of the rainbow (466), and in his 
elaborate investigation of certain cases of the friction 
of fluids (416). But it is more to our present pur- 
pose to observe that he combines this profound and 
technical command of analysis with singular skill in 
the experimental department of optics ;—not merely 
in investigations closely connected with the wave 
theory, and expressible by mathematical formule,’ 
but in those which depend on the judicious question- 
ing of nature by critical experiments not necessarily 
quantitative ;—such, in short, as Newton discusses 
in his Optics ; and indeed since Newton himself occu- 
themselves have been transformed into others, with a 
lower degree of refrangibility, and possessing the cor- 
responding optical properties. In conformity with 
this explanation, Professor Stokes has considered and 
accounted for a number of curious phenomena de- 
scribed by Sir David Brewster and by Sir John Her- 
schel; called by the former “ internal dispersion,” 
by the latter “ epipolic dispersion,” all of which, so 
far as they involve anything peculiar, may be reduced 
to the general principle that certain bodies by their 
action on light have the power of lowering the refran- 
gibility of the rays incident upon them, whether be- 
longing to the visible or invisible spectrum—that is, 
of emitting rays of a lower, while under the excite- 
ment of rays of a higher refrangibility. Hence 
the phenomenon has been termed the “ degradation” 
of light. Mr Stokes has also called it ‘‘ fluores- 
cence,” a word involving no hypothesis, being derived 
from the characteristic action of a certain kind of 
fluor-spar described by Sir David Brewster. 
Many substances, solid and fluid, are found to pos- 
Fluores- 
cence. 
(580.) 
pied the Lucasian chair, there have been perhaps 
few philosophers who have shown so remarkable an 
aptitude for both kinds of research, 
(579.) In 1852 Professor Stokes announced that the re- 
Shows that frangibility of light, which has hitherto been consi- 
pectin my dered its most inherent and invariable quality, may 
fight on in some circumstances be altered. The fundamental 
bechanged. experiment is this: —A beam of solar light is caused 
sess these qualities.» Amongst others, and in the Substances 
highest degree, glass coloured yellow by oxide ike prot 
uranium, called in commerce canary glass, and solu- perty. 
tions of horse-chestnut bark. 
By using sources of light richer than sunlight in (581.) 
the highly refrangible rays ; by also using quartz for nee’ 3! 
the prisms instead of glass (which has the power ment ati 
of absorbing these to a considerable extent)—Pro- a 
ght. 
1 [ may refer to a short but clear paper on the theory and practice of photography by the Rey. W. T. Kingsley, in the Journal 
of the Photographic Society, vol. i., August 1853. Mr Kingsley is one of the most scientific and practical improvers of the art. 
2 This he = also done in a beautiful paper on the effect of polarization in modifying the phenomena of diffraction. Camb. 
Trans., vol. ix. 
* The greater part of these had been detected by Sir D. Brewster, with his usual acuteness and perseverance, in the course of 
his researches on “internal dispersion.” The colouring matter of green leaves is one of the most remarkable. 
