ROTATION OF COLOURED DISCS. 73 
continually enlarge from the circumference to the centre by 
the addition of a given number of degrees (50°) in arithme- 
tical progression. On careful examination of the gray zones 
formed by rotating this scale on a black ground, it will be 
found that each gray zone appears gradually shaded from 
one of its edges, bemg somewhat lighter at its inner edge 
from apposition with a darker gray, and darker at its outer 
edge from contiguity with a lighter gray. These remark- 
able modifications have been clearly demonstrated by M. 
Chevreul.* 
3. Of the Complementary Colours. 
The three primary colours are yellow, red, and blue. 
What is wanting in a given colour to complete this triad is 
called its complementary. The complementary of red, for 
example, is yellow and blue (or green) ; the complementary 
of blue is yellow and red (or orange) ; the complementary of 
yellow is red and blue (or violet), &c. If we gaze steadily 
on a colour for a minute or so, and then direct the eye to a 
contiguous gray surface, the complementary becomes visible. 
These conditions are fulfilled in the following well-known 
and elegant experiment. Place a black wafer on the centre 
of a sheet of emerald-green paper, and over both spread a 
piece of white tissue paper; the wafer no longer appears 
black, but red, tinged, that is, with the complementary of 
the green by which it is surrounded. “In this way colours 
are actually produced by contrast. Thus, a very small dull- 
gray strip of paper, lying upon an extensive surface of any 
bright colour, does not appear gray, but has a faint tint of 
the colour which is the contrast of that of the surrounding 
surface. A strip of gray paper upon a green field, for ex- 
ample, often appears to have a tint of red, and when lying 
upon a red surface a greenish tint; it has an orange-coloured 
tint upon a bright blue surface, and a bluish tint upon an 
orange-coloured surface; a yellowish colour upon a bright 
violet, and a violet tint upon a bright yellow surface. The 
colour excited thus, as a contrast to the exciting colour, 
being wholly independent of any rays of the corresponding 
colour acting from without upon the retina, must arise as an 
opposite or antagonistic condition of that membrane; and 
the opposite conditions, of which the retina thus becomes the 
* «The Principles of Harmony and Contrast of Colours,’ by M. i. 
Chevreul; translated from the French by Charles Martel. Second edition, 
pp- 7—9. 
