PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 48 
of a convenient strength, it came on a little before G. This explains the reason of 
an observation of Sir David Brewster’s, who has remarked that “a beam of light 
that has passed through the esculine solution disperses blue light, but not copiously, 
when transmitted through the quinine solution ; but the beam that has passed through 
quinine is copiously dispersed when transmitted through esculine*.” 
Green Fluor-Spar from Alston Moor. 
32. It is well known that some specimens of fluor-spar exhibit a sort of double 
colour. In particular, a variety found at Alston Moor, which is green when seen 
by transmitted light, appears when viewed in a certain manner of a beautiful deep 
blue. This blue colour seems to have been considered by Sir John Herschel as 
merely superficial. It has been shown however by Sir David Brewster to arise 
from light dispersed in the interior of the crystal, and to have no particular relation 
to the surface. 
The crystal with which the following observations were made was of a fine but not 
intense green when viewed by transmitted light. On viewing a pure spectrum through 
it, there was found to be a dark band of absorption in the red. This band was narrow, 
and by no means intense. The crystal exhibited a copious deep blue by dispersive 
reflexion. 
33. On admitting into the crystal a cone of sunlight formed by a lens of short focus, 
and then analysing the dispersed beam, it was found to consist of a very little red 
followed by a dark interval, then green, faintly fringed below with less refrangible 
colours down perhaps to the orange, then blue, or bluish-green, followed by a great 
deal of indigo or violet. Independently of the gap in the red, the spectrum was not 
quite continuous, for a band of bluish-green, not very broad, was separated by dusky 
bands from the green below and the indigo above. The separate red band and the 
two dusky bands were all so faint as to be difficult to see. 
The dispersed beam was readily proved to be truly dispersed, for it was unpolarized, 
and a pale brown glass cut it off when placed in the first position, although it trans- 
mitted it in a great measure when placed in the second. 
34. When the crystal was examined by the third method, the general result closely 
resembled that produced by sulphate of quinine. The dispersion commenced about 
half-way between G and H, and continued from thence onwards far beyond H. It 
was strongest about H. The fixed lines were seen with beautiful distinctness as dark 
planes in the crystal. The groups II, I, m were quite evident, and n might be seen 
without difficulty. I have even seen some of the fixed lines of the group p. The 
tint of the dispersed light appeared as nearly as possible uniform throughout. The 
distance to which this light could be traced from the surface, did not at all diminish 
so rapidly in this crystal, with an increase in the refrangibility of the incident light, 
* Philosophical Magazine, vol. xxxii. (June 1848), p. 406. 
3 Q 2 
