390 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
over-burnt glasses should be selected for the present purpose, and such are very 
commonly met with. 
Third combination. — In this case everything is the same as in the preceding com- 
bination, except that the fluid is replaced by a glass of a pretty deep blue, coloured 
by cobalt. 
Fourth combination. — In this the principal absorbent is a solution of nitiate of 
copper, and the complementary absorbent a light red or deep orange glass. 
248. In the first combination the darkness is tolerably complete without the use 
of any complementary absorbent, since no visible rays are transmitted except ^iolet 
and some extreme red. The latter are no inconvenience, but rather help to set off 
the tint of the light due to fluorescence. This is, I think, the best combination to 
employ when the fluorescent light is blue, or at least deep blue ; because in that 
case much of the light is lost by absorption in the yellow glass employed in the 
second combination. It has the advantage, too, of allowing the fluorescent light to 
enter the eye without being modified by absorption. Nevertheless no correct estimate 
can be formed of the absolute colour of the fluorescent light without making very 
great allowance for the effects of contrast, especially when the body, instead of being 
isolated as far as possible, is placed on the porcelain tablet. 
The second combination is on the whole the most powerful. The media in this 
case make a very fair approach towards the ideal perfection explained in Ait. 242. 
The darkness is so far complete, or else may easily be made so by increasing a little 
the strength of either absorbent, that if the tablet be written on with ink, and placed 
on the shelf between the media, the writing cannot be read. It forms a striking 
experiment, after having treated the tablet in this manner, to introduce between <^hc 
media a piece of canary glass or a similar medium. The glass is not only luminous 
itself, but it emits so much light as to illuminate the whole tablet, so that the writing is 
instantly visible. In those cases in which the fluorescent light is yellow, orange, or red, 
it is shown a good deal more strongly by this combination than by the preceding. 
The third combination is applicable to the same cases as the second. The blue 
glass answers extremely well, but is not quite so good as the blue fluid. The daik- 
ness is less complete, on account of the red and yellow transmitted by the glass. 
Nevertheless this combination is sometimes useful in observing with a prism, and at 
any rate it may very well be employed by a person who does not happen to have a 
vessel of the proper shape for holding the fluid. 
In the second and third combinations the point of the spectrum at which the 
transparency of the principal absorbent begins, and that of the complementaiy 
absorbent ends, or rather the point which most nearly possesses this character, is 
situated in the blue. Thinking that the fluorescence of those substances which emit 
light of low refrangibility might be better brought out if this point were situated 
lower down in the spectrum, I tried the fourth combination. In this case the media 
have very fairly the required complementary character ; the darkness is pretty com- 
