It has been found necessary to follow the author's nomenclature throughout—T.

Non-perception of blue.

It has not been thought necessary to copy the plates here referred to.—T.

That is (according to the author's statement 150. 151.) both tend to red; the yellow deepening to orange as the comparatively dark medium is thickened before brightness; the blue deepening to violet as the light medium is thinned before darkness.—T.

Plate 2, fig. 1.

The author has omitted the orange and purple in the coloured diagrams which illustrate these first experiments, from a wish probably to present the elementary contrast, on which he lays a stress, in greater simplicity. The reddish tinge would be apparent, as stated above, where the blue and yellow are in contact with the black.—T.

Plate 2, fig. 2

Plate 2, fig. 4

In this case, according to the author, the refracting medium being increased in mass, the appearance of colour is increased, and the displacement is greater.—T.