72 GORHAM, ON THE 
(Yellow + red) = orange or an orange hue. 
(Red 30° + blue 165° + yellow 165°) = neutral. 
(Yellow 15° + blue 15° + red 15° + black 315°) = brown 
(see fig. 8). 
(White + black) = gray, &c., &c. 
2. On Contrast of Tone. 
Black and white are the elements of tone; a colour be- 
comes shaded or of a dark tone by mixture with black, and 
tinted or of a light tone by mixture with white. The mix- 
ture of white and black is gray. Grays themselves are of 
different tones, varying with the relative proportions of their 
elements ; hence there are dark grays and light grays. A 
dark gray appears still darker when viewed in juxtaposition 
with a light gray, and a light gray appears still hghter when 

viewed in contiguity with a dark gray; the altered ap- 
pearance of both is obviously the effect of contrast. This 
may be elegantly illustrated on the colour-top by append- 
ing to a circular disc of black a graduated scale of 
white (fig. 7), and rotating, when a series of gray bands, ar- 
ranged in concentric circles, is immediately formed. The 
graduated scale is composed of segments of circles, which 
