290.
In mediums of similar or nearly similar refracting power, we find the remarkable circumstance that a greater and lesser appearance of colour can be produced by a chemical treatment; the greater effect is owing, namely, to acids, the lesser to alkalis. If metallic oxydes are introduced into a common mass of glass, the coloured appearance through such glasses becomes greatly increased without any perceptible change of refracting power. That the lesser effect, again, is produced by alkalis, may be easily supposed.
291.
Those kinds of glass which were first employed after the discovery, are called flint and crown glass; the first produces the stronger, the second the fainter appearance of colour.
292.
We shall make use of both these denominations as technical terms in our present statement, and assume that the refractive power of both is the same, but that flint-glass produces the coloured appearance more strongly by one-third than the crown-glass. The diagram (Plate 3, fig. 2,) may serve in illustration.
293.
A black surface is here divided into compartments for more convenient demonstration: let the spectator imagine five white squares between the parallel lines a, b, and c, d. The square No. 1, is presented to the naked eye unmoved from its place.
294.