ROTATION OF COLOURED DISCS. 75 
on for a few seconds, the red will soon incline to violet and 
the orange to yellow, for— 
(Red + comp. of orange) = (red + blue) = violet ; 
(Orange + comp. of red) = (orange + green) = yellow- 
orange. 
These effects are most apparent at those parts of the 
coloured bands nearest their line of contact, and become 
gradually weaker at those parts most remote from it. 
The means employed to illustrate these phenomena in the 
colour-top are so simple that it is almost needless to indicate 
them. ‘They consist of a series of rings of colours, affixed 
on differently coloured grounds, and then rotated ; an orange- 
coloured ring is composed of red and yellow in equal por- 
tions (see fig. 10); a violet-coloured ring consists of red and 
blue in the same proportions. <A green ring should result 
in like manner from equal quantities of yellow and blue; but 
as this hue cannot be formed by the rotation of its elements, 
an entire ring of emerald-green is used instead. A yellowish- 
green is formed of yellow and emerald-green in equal parts, 
and a bluish-green of blue and emerald-green. For the 
grounds it is sufficient to use semi-discs of those colours 
which compose them ; thus, two semi-discs of red and blue 
form violet (see fig. 10) ; the shades and tints are constructed 
by adding black or white, as occasion requires. When a ring 
of colour is thus placed on a given ground and rotated, the 
modifications from contrast may be studied with advantage, 
for each colour is formed so expeditiously, that a considerable 
number of illustrations may be examined without trouble or 
fatigue. 
5. The Blending of Colours by soft or Insensible Gradation. 
When a plane, bounded bytwo similar spiral curves (fig. 12), 
having its surface covered with white, black, or any colour, 
is affixed to a disc of a different colour, and rotated, the 
two colours appear to blend in the most delicate manner 
possible, imitating to perfection that kind of mixture known 
by artists as “softening off,’ and exemplified in the blush of 
the cheek, the imperceptible transition from white to red 
seen in the petal of the rose, the bloom on many fruits, the 
golden western sky at sunset losmg itself in an atmosphere 
of blue through a rich neutral, &c. Such combinations are 
exquisitely beautiful. The spiral curve is thus generated :— 
describe a large circle, equal in size to the circumference of 
any of the discs (1 to 6), and a smaller circle near the 
centre (see fig. 11). Between these two circles draw eleven 
segments of circles, at equal intervals, of which the first seg- 
