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.