398         ON THE CHANGfe OF  REFRANGIBILITY   OF  LIGHT.
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 (nee 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 :—
(1)    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 retrun»i-bilities.
(2)    The refrangibility of the incident light is a superior limit