PEOFESSOE STOKES ON THE LONO SPECTEUM OE ELECTEIC LIGHT. 611 
bility, a pair of bluish dots were seen, which were the images of the tips of the electrodes 
exhibited by fluorescence. As the appearance was everywhere the same, on natural 
faces and cleavage planes alike, and the same was observed with colourless felspars 
generally from different localities, it is doubtless a property of the silicate of alumina and 
potash constituting the crystal. Some specimens, it is true, did not show the effect so 
strongly as adularia or moonstone ; but this is easily explained by the greater purity of 
the latter varieties. For the fluorescence extended to a very sensible though small depth 
within the crystal, and yet the rays producing it were cut off by a film of mica much 
thinner than paper. The intense opacity of mica is doubtless due to peroxide of iron, 
which nevertheless forms no more than perhaps 5 per cent, of the mineral. Hence a very 
small percentage of peroxide of iron, or any other impurity having a similar absorbing 
action, would suffice greatly to reduce the quantity of fluorescent light emitted. 
In a concentrated solar beam passed through a suitable absorbing medium, adularia 
did not show the least sign of fluorescence, in which respect it notably differs from com- 
mon glass, such as window-glass. 
The other case of interest relates to a particular variety of fluor-spar found at Alston 
Moor in Cumberland. This variety is very pale by transmitted light, being in part of a 
brownish purple colour, shows a strong blue fluorescence, and is eminently phosphorescent 
on exposure to the electric spark. On presenting such a crystal to the spark passing 
between aluminium electrodes, besides the usual blue fluorescence there is seen another 
of a reddish colour, extending not near so far into the crystal. On receiving on the 
crystal the image of the spark, and moving the crystal from the focus of the invisible 
rays towards the lens, it was soon in best focus for the rays producing the blue 
fluorescence. It had to be moved much nearer to the lens before it came into focus for the 
rays producing the reddish fluorescence, and was then at the distance at which a well- 
deflned image of the tips of the electrodes is formed on the uranium salt ; which proves 
that the reddish fluorescence was produced by the rays belonging to the bright lines 
(considered as a whole) of aluminium of extreme refrangibility. 
The crystal which showed this effect best was externally colourless for about the 
-^th of an inch, which stratum showed no fluorescence when examined in this 
way. Then came one or two strata, parallel to the faces of the cube, showing the ruddy 
fluorescence, and exhausting apparently the rays capable of producing that effect. The 
blue fluorescence extended much deeper, and presented a stratified appearance, as Sir 
David Beewster long ago observed. 
On admitting a pencil concentrated by a quartz lens parallel to and almost grazing a 
face of the cube, so that the rays traversed the colourless stratum, the reddish fluorescence 
was observed in the stratum which produced it to a long distance from the face by which 
the rays were admitted, which demonstrates the transparency of fluoride of calcium for 
the rays of very high refrangibility. 
The property of exhibiting such a well-marked effect under the exclusive influence of 
rays of extreme refrangibility, renders such a crystal a useful instrument of research. 
