524 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
Natural Crystals. 
165. Of natural crystals I have hitherto examined only a small number. Fora 
long time I was occupied almost exclusively with vegetable products, the mineral 
kingdom not appearing promising. However, I have found internal dispersion in 
certain specimens of apatite, arragonite, chrysoberyl, cyanite, and topaz. In all 
these cases the dispersion appeared due, as in the case of fluor-spar, to some sub- 
stance accidentally present in small quantity; so that yellow uranite is at present the 
only natural crystal to the essential constituents of which the property of internal 
dispersion has been found to belong. 
166. Among the minerals just mentioned apatite was the most sensitive, though it 
fell very far short of yellow uranite. That the sensibility was not due to phosphate of 
lime, was plain from the circumstances that a colourless specimen was insensible, and 
that the amount of sensibility was found to be different in different parts of the same 
sensitive specimen. With the exception of the colourless crystal already mentioned, 
all the specimens of apatite examined were of a greenish colour, and all were sensi- 
tive. The dispersed light was something of an orange colour, but was not homo- 
geneous orange. In one specimen it consisted of three distinct bright bands at 
regular intervals. The mode in which the sensibility of this crystal was connected 
with the refrangibility of the incident rays was very peculiar. In arragonite di- 
spersion was found in the transparent specimens examined ; the translucent speci- 
mens were found to be insensible. The dispersed light was of a brownish white 
colour. In the same crystal some parts were insensible and others more or less sen- 
sitive, The portions of equal sensibility were arranged in plane strata, just as in the 
case of fluor-spar, as has been noticed by Sir David Brewster. In a specimen which 
had been cut for showing conical refraction, the strata were in some places perpen- 
dicular to the plane of the optic axes, and in other parts parallel to the line bisecting 
the axes, and inclined to their plane at such an angle that the two directions of the 
strata must have been parallel to two of the commonest lateral faces. Another spe- 
cimen showed strata parallel to an oblique terminal face. The strata are plainly due, 
as Sir David Brewster has remarked with reference to fluor-spar, to some substance 
taken up during crystallization. Accordingly, they preserve a sort of history of the 
growth of the crystal. In a twin crystal of fluor-spar, the direction of the strata in 
that part of the mass which was common to the geometrical forms of both crystals, 
showed to which crystal it really belonged. In fluor-spar the strata are parallel to 
the faces of the cube, at least in the specimens which I have examined, and the same 
has been observed by Sir David Brewster. 
In chrysoberyl, cyanite and topaz, the dispersed light was red or reddish, and was 
too variable to allow of its being attributed to the essential constituents of the 
crystals. In these cases the sensibility was but slight ; indeed in cyanite there was 
only a trace of dispersion when the crystal was examined under great concentration 
of light. 
