PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 39 1 
plete, and the fluorescence of scarlet cloth and similar substances is very well 
exhibited. However, the effect in these cases is shown so well by the second com- 
bination, that, except it be for the sake of varying the experiment, I do not think it 
worth while to employ the fourth combination, more especially as it has the disad- 
vantage of leaving the observer in doubt whether the red or orange light perceived 
constitutes the whole of the fluorescent light, or only that part of it which alone has 
been able to get through the complementary absorbent. 
249. The mode of observation may be altered in various ways which afford pleasing 
illustrations of the theory, though in the regular examination of a set of substances 
It IS best to proceed in a more methodical manner. Thus, if nothing but a violet or 
blue glass or a blue fluid be used as a principal absorbent, and the substances under 
examination be highly sensitive, their appearance will be remarkably changed if the 
coloured medium be transferred from before the hole to before the eyes. Again, if the 
complementary absorbent be made to exchange places with the principal absorbent 
the result will be similar, although the very same media are merely interposed in 
different parts of the compound path of the light from the clouds to'^the eye. If a 
transfer medium be employed, and it be, as has hitherto been supposed, of the same 
general nature as the complementary absorbent, it will not produce much effect when 
it is interposed between the object and the eyes, but when it is placed in the path of 
the rays incident on the object, the fluorescent light will be nearly if not entirely cut 
off. If, however, we take for a transfer medium a glass or fluid having the same 
pneral character as the principal absorbent, the effect will be just the reverse. This 
is strikingly shown in the case of a substance, which, like scarlet cloth, emits a red 
fluorescent light, by taking for a transfer medium a solution of nitrate of copper 
and in the case of turmeric paper or yellow uranite, by taking the same solution, or 
else a blue glass. In the case of the two substances last mentioned, if we take for a 
transfer medium a red solution of mineral chameleon, diluted so as to be merely 
pink, the intensity of the light emitted will, under certain conditions, be not much 
different m the two positions of the medium, because a portion of the active rays in 
one position and a portion of the degraded rays in the other will be absorbed ; but 
the colour of the portion of the emitted light which reaches the eye will be altogether 
different in the two positions of the transfer medium. 
Mode of observing by means of a Prism. 
250. In this method no absorbing medium is required except the principal 
absorbent. The white tablet being laid on the shelf, a slit is first held in such a 
position as to be seen projected against the sky, and the light thus coming directly 
into the eye, after having passed through the principal absorbent, is analysed by a 
prism held in the other hand. The slit is now held so that the tablet is seen through 
It, and the light coming from the tablet is analysed. It will be found that the 
spectrum seen in the first instance is faithfully reproduced, being merely less lumi- 
MDCCCLIII. o 
