Hitherto we have confined our attention to black and white objects relieved on respectively opposite grounds, as seen through the prism, because the coloured edges and borders are most clearly displayed in such cases. We now repeat these experiments with grey objects, and again find similar results.
As we called black the equivalent of darkness, and white the representative of light ([18]), so we now venture to say that grey represents half-shadow, which partakes more or less of light and darkness, and thus stands between the two. We invite the reader to call to mind the following facts as bearing on our present view.
Grey objects appear lighter on a black than on a white ground ([33]); they appear as a light on a black ground, and larger; as a dark on the white ground, and smaller. ([16].)
The darker the grey the more it appears as a faint light on black, as a strong dark on white, and vice versâ; hence the accessory images of dark-grey on black are faint, on white strong: so the accessory images of light-grey on white are faint, on black strong.
Grey on black, seen through the prism, will exhibit the same appearances as white on black; the edges are coloured according to the same law, only the borders appear fainter. If we relieve grey on white, we have the same edges and borders which would be produced if we saw black on white through the prism.—[Note O.]