540 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
refrangibility as to place them among the groups of fixed lines m, n, or a little beyond, 
since when a solution of guaiacum is examined in the solar spectrum, it is found that 
that is the region in which the blue dispersed light is produced. The blue light 
dispersed by a solution of guaiacum may also be seen by using the blue flame of 
sulphur burning feebly. The poverty of the flame of a spirit-lamp, not only with 
respect to visible rays, but also with respect to invisible rays, except those of very 
high refrangibility, accounts for the circumstance that it does not exhibit, or at least 
hardly at all exhibits, the blue light dispersed by fluor-spar. 
Mode of determining, hy means of the light of a spirit-lamp, the transparency of 
bodies with respect to the invisible rays of high refrangibility . 
201. If the body be a solid, and be bounded by parallel surfaces, its transparency 
with regard to these rays is easily tested. For this purpose it is sufficient to hold 
the flame of a spirit-lamp a little way above the surface of a weak solution of sulphate 
of quinine contained in an open vessel in a dark room, and then, placing the eye so 
as to see the dispersed light in projection, alternately to interpose and remove the 
plate to be examined. 
202. On examining in this way various specimens of glass, I found none which 
did not show evident defects of transparency. The purest speeimens of plate-glass 
appeared, I think, to be the least defective. I cannot say whether the observed 
defects of transparency were due to the essential ingredients of the glass, or to acci- 
dental impurities. It is possible that glass made with chemically pure materials 
might be transparent*. I believe that a mere trace of peroxide of iron, or of sul- 
phuret of soda or potassa, would be sufficient to impair materially the transparency 
of glass with respect to these rays, and such impurities are very likely to be present. 
Quartz, however, appeared to be perfectly transparent, the active rays passing 
through the thickness of one or two inches, whether parallel or perpendicular to the 
axis, without any perceptible loss. The contrast between quartz and mica was very 
striking, for a plate of mica no thicker than paper produced a very sensible diminu- 
tion in the illumination. 
203. For the purpose of observing fluids, I procured two vessels consisting of sec- 
tions of a wide glass tube, about an inch long, closed at one end with a disc of quartz. 
I shall call these for brevity quartz vessels, though of course the bottom is the only 
part in which there is any occasion to use quartz. When a fluid is to be examined 
it is poured into a quartz vessel, and then the vessel with its fluid contents is ex- 
amined in the manner of a solid plate, as described in Art. 201. On account of the 
perfect transparency of quartz, the fluid is as good as suspended in air. When a 
* Some specimens of glass belonging to Dr. Faraday’s experiments, which from the absence of colour and 
of internal dispersion seemed hopeful, could not be examined for transparency, on account of their irregular 
figure ; and as they were only lent to me by a friend, I did not feel myself at liberty to get them cut and 
polished. 
