496 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
stances, some of them, perhaps, coloured, so that the colour of the solution may be 
very different from what it would be if the sensitive principle were present alone. 
71. The law mentioned at the beginning of art. 63 did not seem very applicable 
to archil when the fluid was merely diluted with water. But when the orange- di- 
persing and green-dispersing principles were obtained, as it would appear, more 
nearly in a state of isolation, by means of ether and water, the law was found to be 
obeyed. Thus, when the ethereal solution which exhibited the orange dispersion 
and little else was examined by the third method, the disperson was found to com- 
mence with a tail of light followed by a dark tooth, indicating the position of a band 
of absorption. When the light transmitted by a certain thickness of this fluid was 
subjected to prismatic examination, it was found to consist of red followed by some 
orange, when the spectrum was cut off with unusual abruptness. After a broad dark 
interval came the most refrangible colours faintly appearing. Those solutions which 
exhibited a copious dispersion of green gave, in addition to a band obliterating the 
yellow, a very distinct band separating the green from the blue. A similar band, 
but by no means distinct, might be seen in archil merely diluted ; and it is particularly 
to be observed that this band, which occurred a little above the point of the spectrum 
where the green dispersion commenced, became more conspicuous when the green- 
dispersing principle was present more nearly in a state of isolation. 
72. Two portions of litmus were treated, one with ether and the other with alcohol, 
which were allowed to remain in contact with the solid. Both extracts, but espe- 
cially the latter, were highly sensitive, exhibiting dispersions of orange and green 
similar to archil, and due apparently to the same sensitive principles. The ethereal 
extract dispersed chiefly orange, while the alcoholic extract dispersed orange and 
green in nearly equal quantities. The latter extract exhibited a remarkably copious 
dispersive reflexion of a colour nearly that of mud, and was altogether one of the 
strangest looking fluids that I have met with. On viewing it in such a manner that no 
transmitted light entered the eye, one might almost have supposed that it was muddy 
water taken from a pool on a road. But when the bottle containing it was held be- 
tween the eye and a window the fluid was found to be perfectly clear, and of a beautiful 
purple colour. 
Canary Glass. 
73. Among media which possess the property of internal dispersion in a high 
degree. Sir David Brewster mentions a yellow Bohemian glass, which dispersed a 
brilliant green light. This led me to seek for such a glass, and it proved to be pretty 
common in ornamental bottles and other articles. The colour of the glass by trans- 
mitted light is a pale yellow. Its ornamental character depends in a great measure 
upon the internal dispersion, which occasions a beautiful and unusual appearance in 
the articles made of it. The commercial name of the glass is canary glass. The 
following observations were made with a small bottle of English manufacture. 
74. When the sun’s light was admitted without decomposition the dispersed beam 
