126 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



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. 1 



(576.) If anything were wanting to show the impossibility 

 Importance o f separating the scientific arts from the history of 



to 're Mc science itself > ifc W0uld be the case before us " The 

 art. 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. 



(577.) So intimately connected with the discovery of the 

 Chemical Chemical Rays of the spectrum, is that of revealing 

 rays ren- ^j ie j r existence (at least the existence of rays of the 



dered vi- A J 



sible. spectrum equally refrangible with those possessing 

 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. 

 (578.) Mr GEORGE GABRIEL STOKES, a fellow of Pembroke 

 Professor Q o \\ e gQ i Cambridge, and senior wrangler in 1841, 

 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 formula, 2 

 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- 

 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- 

 * ibiHt 'of " ^ ere d *ts most inherent and invariable quality, may 

 light may in some circumstances be altered. The fundamental 

 be changed, experiment is this : A beam of solar light is caused 



to form a pure and highly dispersed spectrum by Fluores- 

 passing through two or three prisms in succession, cence< 

 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 

 (usually) invisible spectrum, are marked by obscure 

 planes traversing the mass of diffuse light. When 

 the light thus emanating from the interior of the 

 quinine is examined by a prism, it is found to con- 

 sist of rays of various colours and refrangibilities 

 corresponding to those of the ordinary spectrum ; 

 from which Mr Stokes concludes that, since only vio- 

 let rays and the invisible rays beyond them could have 

 furnished or excited this emanation? the rays of light 

 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- (580.) 

 sess these qualities. 3 Amongst others, and in the Substances 

 highest degree, glass coloured yellow by oxide of]^!^ 2 

 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 ^ e - mark ' 

 the prisms instead of glass (which has the power ment '^-^ 



of absorbing these to a considerable extent) Pro- electric 



light. 



1 I may refer to a short but clear paper on the theory and practice of photography by the Rev. W. T. Kingsley, in the Journal 

 of the Photographic Society, vol. i., August 1853. Mr Kinsley is one of the most scientific and practical improvers of the art. 



2 This he has also done in a beautiful paper on the effect of polarization in modifying the phenomena of diffraction Camb. 

 Trans., vol. ix. 



3 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. 



