PEOPESSOE STOKES ON THE LONG SPECTEIJM OF ELECTEIC LIGHT. 613 
whole, I am disposed to think that quartz, if it were rigorously pure, would be transpa- 
rent. We see at any rate how difficult it is to draw certain conclusions respecting the 
transparency or opacity of a substance which, in the state of purity in which it may be 
obtained, shows only a slight defect of transparency. 
I tried reflecting the rays from the spark by a flne Munich grating, but the light was 
far too faint to be of any use. Possibly a large and very closely ruled plane speculum, 
with a concave spec\ilum instead of a lens, might give light which it would be possible 
to observe. But at present I have not found any sufficiently marked effects referable to 
rays of still higher refrangibility to make it worth trying. 
The same crystal which showed the reddish fluorescence was eminently phosphorescent, 
with a blue colour. The phosphorescence, like the fluorescence, was arranged in strata 
parallel to the faces of the cube, and, like the reddish but unlike the blue fluorescence, 
was not perceptible beyond a moderate distance from the surface at which the exciting 
rays had entered. On forming an image of the discharge by the 2'5-inch lens, focusing 
the crystal for the rays producing the reddish fluorescence, flxing it there, and breaking 
the circuit after the induction coil had worked for a little while, a dart of blue phos- 
phorescent light was seen in the crystal at the focus of the lens. On focusing for the 
rays most efficient in producing the blue fluorescence, the reddish was diffused over a 
broad portion of the strata producing it ; and on repeating the above experiment in this 
position of the crystal, the blue phosphorescence was seen similarly diffused. This 
shows that the rays of extremely high refrangibility are those most efficient in producing 
the blue phosphorescence. 
[We may suppose that the blue fluorescence, the reddish fluorescence, and the blue 
phosphorescence are due to the action of the assemblage of heterogeneous exciting rays 
on the same substance (doubtless some impurity taken up during crystallization), or on 
two or three distinct substances. The blue fluorescence is produced abundantly at a depth 
within the crystal at which the two other eflects are invisible ; but this alone is no proof 
of a diversity in the nature of the substance acted on, because the rays producing the two 
latter effects would have been absorbed before arriving at such a depth. Hence it is 
among the early strata, in crossing which rays capable of producing each of the three 
effects are still vigorous, that evidence must be sought, in the coincidence or non-coinci- 
dence of the strata in which the three effects are respectively perceived, of the probable 
identity or certain diversity of nature of the substance acted on. At the time when this 
paper was read I fancied I had observed slight discrepancies as to coincidence in the 
strata. But a renewed examination, in which a larger number of specimens were observed, 
leads me to regard the fancied discrepancies as too doubtful to rely upon and to over- 
power the increasing weight of evidence on the other side. 
The blue fluorescence may be observed in the early strata (which ordinarily, at least 
with electrodes of aluminium and several other metals, show a red) by absorbing the 
more refrangible of the exciting rays by a suitable plate of quartz, or else by substituting 
for aluminium some metal, such as magnesium, which is poor in rays of extreme refran- 
