612 PEOFESSOE STOKES OK THE LONO SPECTEHM OF ELECTEIC LIOHT. 
Several other metals besides aluminium show the reddish fluorescence ; but none of those 
examined showed it so well, partly because it is evidently produced more copiously by 
aluminium electrodes, and partly because it is less masked by the blue fluorescence, 
the spectrum of aluminium being rather wanting in brightness until the region of extreme 
refrangibility is reached. 
Tf the crystal be held near the electrodes, and observed while their distance changes, it 
will be found that on passing from the greatest striking distance the reddish fluorescence 
decidedly improves. On still further diminishing the distance between the electrodes, 
the reddish fluorescence appears still to increase ; though whether this is a real absolute 
increase or only an increasing preponderance over the blue, it is not easy in this way to 
say for certain. Hence the copiousness of rays of high refrangibility increases at flrst, and 
continues to increase relatively if not absolutely. It is supposed that the jar is sufii- 
ciently large to prevent the discharge from degenerating into what will be presently 
described as the arc discharge. 
If the crystal be held close to the contact-breaker when the secondary terminals are 
separated, and the effect be compared with that of the secondary discharge (a jar being 
in connexion, as has been supposed all along), the electrodes being of platinum for fair- 
ness of comparison, it will be found that the proportion of rays of extremely high 
refrangibility is decidedly greater for the spark at the contact-breaker than for the 
secondary discharge. 
On forming by the 2'5-inch lens an image of the spark from aluminium electrodes, 
and placing a crystal, such as that above mentioned, in the focus of the rays producing 
the reddish fluorescence, it is easy to determine the transparency or opacity of substances 
for those rays, the alteration of the focus by the introduction of a thick plate being of 
course borne in mind, and the crystal moved accordingly. The rays forming the image 
have had to pass only through air, and through a very small thickness of quartz, before 
reaching the crystal. In this way I have found that even quartz itself in very moderate 
thickness is opaque for these rays ; but difierent specimens, or difierent parts of the same 
specimen, vary in this respect. I possess a large plate 0'42 inch thick, cut perpendi- 
cular to the axis of the crystal, which is generally transparent, but is slightly brownish 
on one side, to the distance of about half an inch from the face of the hexagonal prism. 
The colourless part of this plate, beyond a little distance from the brownish part, is 
opaque for the rays in question while the brownish part is nearly transparent. It may 
be inferred that the colourless part contains a minute quantity of some impurity capable 
of absorbing these rays, which does not exist, at least to the same extent, in the brownish 
part, although the latter is not perfectly pure silica, as is shovm by its colour. On the 
* It should be mentioned that this part contains those delicate, definitely directed, elongated laminte or 
crystals, hardly visible except in a beam of sunlight, which are called by practical opticians “ blue shoots.” 
An examination of a number of cut pieces of quartz lent me by Mr. Daekee confirms me in the suspicion 
that such crystals are more defective in transparency than other colourless specimens for the rays of extreme 
refrangibility. — July 1862. 
