Fis. 1. 
392 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
nous as must necessarily happen. At least, this was the case in those tablets which 
I have examined ; and in this way each observer ought to test for himself the a e 
he proposes to employ. After having been thus tested, the tablet may be use as 
stiindard of coiripa.i’ison. -u j 
Suppose now that it is wished to examine a slip of turmerm paper, or a riband r 
otherTimilar object. The object is laid on the tablet, and the slit held immediately 
in front of it, in such a manner that one part, suppose the central Port'™- 
I seen proie;ted on the object, and the remainder on the tablet. The hght coming 
throTgh the slit is then ailysed by the prism, and the fluorescence, tf any of the 
object is indicated by light appearing in those regions of the spectrum in w ic , in 
the case of the light scattered by the tablet, there is nothing but darkness. 
Occasionally in these observations a blue glass is preferable to a solution of he 
ammoniaco-sulpbate of copper, because the extreme red and the greenish le 
bands transmitted by the glass, while too faint to interfere with the fluoiescent g , 
are useful as points of reference. 
251. The general appearance of the spectrum 
in this mode of observation may be gathered from 
the accompanying figures, of which the first repre- 
sents turmeric paper seen under the blue glass, 
and the second represents a mass of crystals of 
nitrate of uranium seen under the copper solution. 
In fig. 1, RR', YV are the red and yellow bands 
transmitted by the glass, which are seen equally 
in the light scattered by the tablet and that 
scattered by the paper. BVB'V' is the blue and 
violet light transmitted by the glass. Of this a 
considerable portion, especially in the mote lefran- 
gible part, is absorbed by the turmeric paper, 
which on the other hand emits a quantity of red, — 
yellow, and green light, not found among the incident rays. Fig. 2 sufficient y 
explains itself. In this case the fluorescent light is decomposed by the prism mto 
bright bands, of which six may be readily made out. No blue or violet light enters 
the eye from the part of the slit which is seen projected on the mass of crystals, 
except where a crystalline face happens to be situated in such a position as to reflect 
the light of the sky into the eye, as represented in the figure. In the case of a sub- 
stance so highly sensitive as nitrate of uranium, and which does not, like a slip ol 
paper lie flat on the tablet, the spectrum of the fluorescent light in reality extends, at 
least on the side next the window, though with less intensity, to some distance be^iid 
the part of the slit which corresponds to the object, because the tablet is lighted up 
by the rays emitted by the object ; but this is not represented in the figure. 
252. The mode of using the prism just explained is that by which the plienomeiiou 
Fiff. -2. 
