530 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
false dispersion is almost always exhibited along with true dispersion, the quantity of 
the former seems to have no relation to the quantity of the latter, but does seem to 
have relation to the greater or less degree of clearness which we should be disposed 
to attribute to the fluid. 
180. The phenomenon of false internal dispersion seems to admit of being applied 
as a chemical test to determine whether or not precipitation takes place. Thus, if a 
little tincture of turmeric be greatly diluted with alcohol, and then water be added, 
a yellow fluid is obtained which appears to be perfectly clear, exhibiting no sensible 
opalescence ; but the occurrence of a copious false dispersion when the fluid is 
examined by sunlight, reveals at once the existence of suspended particles, though 
they are too minute to be seen individually, or even to give a discontinuous appear- 
ance to the falsely dispersed beam. Although such a precipitation could not, I 
suppose, be used as a means of mechanical separation, it might still be useful as 
pointing out the possibility of an actual separation under different circumstances as 
to strength of solution, &c. 
181. One of the best instances of false dispersion that I have met with, best, that 
is, in forming a most excellent imitation of true dispersion, occurred in the case of 
a specimen of plate-glass which was made, as I was informed, with a quantity of 
alkali barely sufficient. This glass, which was very slightly yellowish brown, when 
viewed edgeways by transmitted light, had a bluish appearance when viewed pro- 
perly, strongly resembling that of a decoction of the bark of the horse-chestnut, 
diluted with water till the dispersed light is no longer concentrated in the neigh- 
bourhood of the surface. But when the glass was examined by sunlight, the polariza- 
tion of the dispersed beam, and the identity of its refrangibility with that of the 
incident light, showed that this was merely an instance of false dispersion. Another 
very good example of false dispersion is afforded by chloride of tin dissolved in a 
very large quantity of common water. 
182. When a horizontal beam of falsely dispersed light is viewed from above, in a 
vertical direction, and analysed, it is found to consist chiefly of light polarized in 
the plane of reflexion. It has often struck me, while engaged in these observations, 
that when the beam had a continuous appearance, the polarization was more nearly 
perfect than when it was sparkling, so as to force on the mind the conviction that it 
arose merely from motes. Indeed, in the former case, the polarization has often 
appeared perfect, or all but perfect. It is possible that this may in some measure 
have been due to the circumstance, that when a given quantity of light is diminished 
in a given ratio, the illumination is perceived with more difficulty when the light is 
uniformly diffused than when it is spread over the same space, but collected into 
specks. Be this as it may, there was at least no tendency observed towards polariza- 
tion in a plane perpendicular to the plane of reflexion, when the suspended particles 
became finer, and therefore the beam more nearly continuous. 
183. Now this result appears to me to have no remote bearing on the question of 
