result, can never be arrested.
III.
GREY SURFACES AND OBJECTS.
35.
A moderate light is essential to many chromatic experiments. This can be presently obtained by surfaces more or less grey, and thus we have at once to make ourselves acquainted with this simplest kind of middle tint, with regard to which it is hardly necessary to observe, that in many cases a white surface in shadow, or in a low light, may be considered equivalent to a grey.
36.
Since a grey surface is intermediate between brightness and darkness, it admits of our illustrating a phenomenon before described (29) by an easy experiment.
37.
Let a black object be held before a grey surface, and let the spectator, after looking steadfastly at it, keep his eyes unmoved while it