PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 525 
Coloured Glasses. 
167. Besides canary glass, I have examined the common coloured glasses, including 
that coloured by gold, but with one exception have not met with any example in 
which the sensibility observed appeared to have any connexion with the colouring 
matter. The paler glasses exhibited a little internal dispersion, because the colour 
was not sufficiently intense to mask the dispersion which a common colourless glass 
would exhibit. 
168. The exception occurred in the case of the pale brown glass, which has been 
already mentioned in connexion with my first experiment. This glass dispersed a 
red light under the influence of the highly refrangible rays. The colour of the light 
was not pure prismatic red, but red was predominant. A similar dispersion, due 
apparently to the same cause, was observed in the case of one of the common reddish 
brown German wine bottles. The sensibility of these glasses appears to be due to 
an alkaline sulphuret. A bead purposely coloured in this manner was in fact found 
to disperse a red light like the glasses. Moreover, in the confused masses obtained 
by fusing sulphate of soda and sulphate of potash on charcoal before the blowpipe, 
certain portions were found which dispersed a red light, and that prett}'^ copiously 
for an inorganic substance. A similar dispersion was observed among the products 
obtained by fusing together sulphur and carbonate of potash, while other parts of the 
confused mass exhibited dispersion of a different kind. It seems plain that among 
the combinations of sulphur with the alkalies sensitive compounds exist, but what 
they are I have not examined. 
Cautions with respect to the discrimination between true and false internal dispersion. 
169. In the early part of this paper certain tests were given for distinguishing 
between true and false internal dispersion in a fluid. But it requires some experience 
in observations of this kind to be able readily to decide, and a too rigid adherence 
to one of the tests to the exclusion of the others might lead to error. 
The first test relates to the continuous appearance of a truly dispersed beam. But 
sometimes solid particles exist in mechanical suspension, which are so fine and so 
numerous, that this test alone might lead the observer to mistake a falsely for a 
truly dispersed beam. On the other hand, if a fluid which itself alone exhibits no 
internal dispersion, true or false, hold solid particles in what is obviously mere 
mechanical suspension, we must not immediately conclude that the medium, taken 
as a whole, is incapable of changing the refrangibility of any portion of the light 
incident upon it. For we have seen that the fluid state is not in the least degree, 
essential to the exhibition of sensibility, and of course a fluid will serve as well as 
anything else for the mere mechanical support of a sensitive substance. 
170 . Thus lycopodium is very sensitive, as appears by examining the powder in a 
linear spectrum. Accordingly, I found that when a little lycopodium was mixed 
with water, and the whole medium was examined by the fourth method, it displayed 
