498 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILTTY OF LIGHT. 
alone, which contains not a ray having the same refrangibility as any one of the 
dispersed rays, the dispersion would be exhibited in full perfection. 
Common Colourless Glasses. 
78 . Sir David Brewster states that he has met with many specimens, both of 
colourless plate and colourless flint glasses, which disperse a beautiful green light. 
All the colourless glasses which I have examined dispersed light internally to a 
greater or less extent, with the exception of some few specimens belonging to 
Dr. Faraday’s experiments. A beautiful green seems to be the commonest tint of 
the dispersed beam, and this I have found in wine glasses, decanters, apothecaries’ 
bottles, pieces of unannealed glass, &c. ; also in many specimens of plate and 
crown glass. The green Was generally of a finer tint than that dispersed by the 
canary glass, but was not near so copious. On analysis it was found to consist 
usually of red and green separated by a dark band, or rather a minimum of bright- 
ness. Those specimens which were examined by the third and fourth methods were 
found to exhibit a little false dispersion, produced chiefly in the brightest part of the 
spectrum, but the greater part was true dispersion. This dispersion was produced 
chiefly by a rather narrow band, comprising the fixed line G, where there appeared 
to be a remarkable maximum of sensibility. The line G lay a little above the lower 
limit of the band. Below the band dispersion also took place, though not near so 
copiously, and there appeared to be another maximum of sensibility some way further 
down in the spectrum ; but above the band dispersion almost entirely ceased of a 
sudden; a very unusual circumstance when the active and the dispersed light are 
well separated in refrangibility. The position of the band in the spectrum, and the 
distribution of the illumination in it, which are very peculiar, were the same in all the 
specimens which were sufficiently sensitive to admit of being examined by the third 
method, but the tint of the dispersed light was not quite the same. 
79. Orange-coloured glasses are frequently met with which reflect from one side, 
or rather scatter in all directions, a copious light of a bluish-green colour, quite dif- 
ferent from the transmitted tint. In such cases the body of the glass is colourless, 
and the colouring matter is contained in a very thin layer on one face of the plate. 
The bluish green tint is seen when the colourless face is next the eye. As this phe- 
nomenon was supposed by Sir John Herschel to offer some analogy with the re- 
flected tints of fluor-spar and a solution of sulphate of quinine, I was the more de- 
sirous of determining the nature of the dispersion. It proved on examination to be 
nothing but false dispersion, so that the appearance might be conceived to be pro- 
duced by an excessively fine bluish-green powder contained in a clear orange stratum, 
or in the colourless part of the glass immediately contiguous to the coloured stratum. 
The phenomenon has therefore no relation to the tints of fluor-spar or sulphate of 
quinine. It is true that the very same glass which displayed a superficial reflexion 
of bluish green, when examined by condensed sun-light exhibited also, in its colour- 
