374 Dr. Wollaston’s Method of examining 
great refractive density. In the refractions from this spar into 
flint glass, and into all oils or resins, I believe, without excep- 
tion, the colours are seen reversed. 
Rock crystal likewise disperses so little, that it exhibits the 
colours reversed, when it is in contact with many substances of 
less refractive power than itself. I have tried it with Dutch 
plate-glass, with Canada balsam and balsam of capivi, with 
many oils essential and expressed, and have found the colours 
in all these cases reversed. 
By solutions of metallic salts, a great variety of such appear- 
ances may be produced. Most of these compounds have a highly 
dispersive power; and many of them may be rendered suffi- 
ciently dense to occasion reversion, even when applied to flint- 
glass. In a more dilute state, they may be used with crown- 
glass, or plate-glass, to produce the same effect. i\.nd since, 
when further diluted by a less dispersive medium, they will also 
present an appearance of colourless refraction, we may, by 
examining the degree of dilution necessary for that purpose, 
compare the dispersive powers of any ingredients contained in 
them, and may gradually extend our knowledge of this property 
to the elements of any bodies, however compounded. 
As a specimen of the method, I have in this way compared a 
few solutions of metals, and of other substances, that were each 
diluted till the limit of reflection appeared void of colour, when 
they were in contact with a rectangular piece of plate-glass; 
and, in the table which follows, I have expressed their refractive 
powers in that state of dilution, as nearly as the eye can discern 
the disappearance of colour. 
