PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 393 
of the change of refrangibility is most strikingly illustrated ; but in the actual examina- 
tion of substances the chief use of the prism is to determine, in the case of substances 
which are sufficiently sensitive to admit with advantage of such a mode of observation, 
the composition of the fluorescent light. For this purpose it is often better to isolate 
the object by placing it on black velvet. This is especially the case with very minute 
crystals, or other objects, which are best placed on black velvet, and viewed through 
the prism as a whole, no slit being required. 
Examples of the application of the preceding methods of observation. 
253. The peculiar properties of paper washed with tincture of turmeric or stramo- 
nium seeds, of yellow uranite, and other highly sensitive substances, come out in a 
remarkable manner under the modes of examination described in this paper. I need 
not say that such is the case with solutions of sulphate of quinine, or horse-chestnut 
bark, or other clear and highly sensitive media, seeing that in this case the appearance 
due to fluorescence is obvious to com mon observation. If a piece of horse-chestnut bark 
be put to float in a glass of water close to the hole covered by the principal absorbent, 
the appearance of the descending streams of solution of esculine is very singular and 
beautiful. My present object is however rather to illustrate the power of these methods 
by their application to substances which stand much lower in the scale. 
By the use of absorbing media alone, as well by a principal absorbent and a prism, 
I have been able to detect without difficulty the sensibility of white paper on a day 
of continuous clouds and rain. Even cotton wool, which stands very much lower in 
the scale, is shown by the use of absorbing media with ordinary daylight to be sensitive. 
In the case of such substances as bone, ivory, white leather, the white part of a quill, 
which stand much higher in the scale, the most inexperienced observer could hardly 
fail instantly to detect the fluorescence. All plates of colourless glass which I have 
examined, and other pieces which were of such a shape as to admit of being looked 
into edgeways to a considerable depth, were found by the second combination to be 
sensitive. Crystals of sulphate of quinine, which may be readily prepared from the 
disulphate of commerce, show their fluorescence extremely well by the first combina- 
tion. These crystals are much less sensitive than their solution, and the light which 
they emit is of a much deeper blue. It must in reality be of a very deep blue colour ; 
for it nearly matches the fluorescent light of fluor-spar, although when the crystals 
are viewed under the violet glass the tint in both cases appears comparatively pale, 
from contrast with the violet. A solution of nitrate of uranium on the other hand 
has only a low degree of sensibility compared with the crystals of that salt. If a 
drop of the solution be placed on the porcelain tablet when the hole is covered with 
the deep blue copper solution, it appears comparatively dark, because much more 
illumination is lost by the absorption of the indigo and violet than is gained by the 
fluorescence of the solution. But when the tablet is viewed through the comple- 
mentary absorbent, the solution is seen to be more luminous than the tablet, and to 
emit yellow rays, which are not found in the incident light. 
3 F 2 
