PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITV OF LIGHT. 473 
17. On refracting the whole system sideways through a prism of moderate angle 
held in front of the eye, the fixed lines became confused, and the finer ones disap- 
peared. The edges of the broad bands H were tinged with prismatic colours, like 
the edges of two slips of black velvet placed on a sheet of pale blue paper, and 
viewed through a prism. This experiment exhibits the compound character of the 
dispersed light, notwithstanding the perfect homogeneity of the incident light. 
18 . The third method of observation is well adapted to bring into view the 
variation in the absorbing energy of the medium corresponding to a variation in the 
refrangibility of the incident rays. When the eye is placed vertically over the vessel 
containing the solution, so that the dark planes corresponding to the fixed lines of 
the spectrum are projected into dark lines, of which the length is not exaggerated by 
obliquity, the boundary of the dispersed light is projected into a curve, which serves 
to represent to the eye the relation between the absorbing power of the medium and 
the refrangibility of the incident light. This curve is not exactly that which Sir John 
Herschel has treated of in the theory of absorption, and considered as the type of 
the absorbing medium to which it is applied, but nevertheless it serves much the same 
purpose. It is true, that, independently of any change in the absorbing energy of the 
medium, an increasing faintness in the dispersed light would produce to a certain 
extent an approximation of the curve to its axis ; but practically, in the case of sul- 
phate of quinine, as well as in a great many others, the appearance is such as to leave 
no doubt as to the existence of a most intense absorbing energy on the part of the 
medium with respect to rays of very high refrangibilities^. 
In the case of a solution of sulphate of quinine of the strength of one part of the 
disulphate to 200 parts of acidulated water, it has been already stated that a portion 
of the rays which are capable of producing dispersed light passed across a thickness 
of 3 inches. On forming a pure spectrum, the fixed line H was traced about an inch 
into the fluid. On passing from H towards G, the distance that the incident rays 
penetrated into the fluid increased with great rapidity, while on passing in the con- 
trary direction it diminished no less rapidly, so that from a point situated at no great 
distance beyond H to where the light ceased, the dispersion was confined to the im- 
mediate neighbourhood of the surface. When the solution was diluted so as to be 
only one-tenth of the former strength, a conspicuous fixed line, or rather band of 
sensible breadth, situated in the first group of fixed lines beyond H, was observed to 
penetrate about an inch into the fluid. On passing onwards from the band above- 
mentioned in the direction of the more refrangible rays, the distance that the incident 
rays penetrated into the fluid rapidly decreased, and thus the rapid increase in the 
absorbing energy of the fluid was brought into view in a part of the spectrum in 
* I should here remark, that, after the researches described in this paper had far advanced, I met accidentally 
with a passage in the Comptes Rendus, tom. xvii. p. 883, in which M. Ed. Becqueeel mentions a solution of 
acid sulphate of qmnine as a medium eminently remarkable for its absorbing power with respect to the rays 
more refrangible than H. 
3 p 2 
