556 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
to observe the changes which in that case take place, but poured off when the strength 
of the solution is thought sufficient. Also, the solution when out of use must be kept 
in the dark. 
A weak solution of the bark of the horse-chestnut. 
A weak solution of sulphate of quinine, i. e. a solution of the common disulphate 
in very weak sulphuric acid. Various other salts of quinine are nearly if not quite 
as good. 
Fluor-spar (a certain green variety). 
Red sea-weeds of various shades ; a solution of the red colouring matter in cold 
water-. If a solution be desired, a sea-weed must be used which has never been 
dried. Sometimes even a fresh sea-weed will not answer well. 
A solution of the seeds of the Datura stramonium in not too strong alcohol. 
Various solutions obtained from archil and litmus (see Arts. 65 to 72). 
A decoction of madder in a solution of alum. 
Paper washed with a pretty strong solution of sulphate of quinine, or with a solution 
of stramonium seeds, or with tincture of turmeric. The sensibility of the last paper 
is increased by washing it with a solution of tartaric acid. This paper ought to be 
kept in the dark. 
A solution, not too strong, of guaiacum in alcohol. 
Safflower-red, scarlet cloth, substances dyed red with madder, and various other 
dyed articles in common use. 
Many of the solutions here mentioned are mixtures of various compounds. Of 
course if the sensitive substance can be obtained chemically pure it will be all the 
better. 
Conclusion. 
240. The following are the principal results arrived at in the course of the 
researches detailed in this paper : — 
fl.) In the phenomenon of true internal dispersion the refrangibility of light is 
changed, incident light of definite refrangibility giving rise to dispersed light of 
various refrangibilities. 
(2.) The refrangibility of the incident light is a superior limit to the refrangibility 
of the component parts of the dispersed light. 
(3.) The colour of light is in general changed by internal dispersion, the new 
colour always corresponding to the new refrangibility. It is a matter of perfect 
indifference whether the incident rays belong to the visible or invisible part of the 
spectrum. 
(4.) The nature and intensity of the light dispersed by a solution appear to be 
strictly independent of the state of polarization of the incident rays. Moreover, 
whether the incident rays be polarized or unpolarized, the dispersed light offers no 
traces of polarization. It seems to emanate equally in all directions, as if the fluid 
were self-luminous. 
