The author has before observed that colour is a degree of darkness, and he here means that increase of darkness, produced by transparent mediums, is, to a certain extent, increase of colour.—T.

It has been thought unnecessary to give all the examples in the plate alluded to, but the leading instance referred to in the next paragraph will be found in plate 3, fig. 1. The grey square when seen through a prism will exhibit the effects described in par. 257.—T.

Plate 3, fig. 1. The author always recommends making the experiments on an increased scale, in order to see the prismatic effects distinctly.

Neither the description of the apparatus nor the recapitulation of the whole theory, so often alluded to by the author, were ever given.—T.

Plate 3. fig. 1.

The grey square is introduced in the same plate, fig. 1, above the coloured squares.

This description of the apparatus was never given.

Plate 4. fig. 1.