ROTATION OF COLOURED DISCS. ae 
distinct image; and owing to the rapidity of the vibrations, 
a whole circle of images is thus portrayed on the retina of 
the eye before the image from the first vibration is effaced. 
In like manner, the colours of the disc a, which are per- 
ceived only as they are transmitted through the open designs 
in the disc B, appear in their primitive purity or unmixed 
state, the colour in one sector being reflected through a 
given ray of the pattern before the arrival of the colour from 
that sector by which it is immediately succeeded. Hence, 
both pattern and colours appear multiplied, thus producing 
the combinations. 

From the construction of the instrument, about five revo- 
lutions of the disc a occur to one single revolution of the 
disc 8. When these relative velocities are maintained, five 
groups of all the colours distributed on the colour-disc are 
seen occurring in the order of their arrangement on the 
disc, and repeated in perfect symmetry in the various open- 
ings of the patterns (see fig. 14, which represents a non- 
rotating disc, and fig. 15, the same during rotation). In 
this way the most beautiful variations may be effected by 
using different colours in various proportions on the disc a ; 
for however numerous the colours, each colour is reflected 
through its proper opening in the disc B, at a given interval 
of space and time, without the slightest irregularity or confu- 
s10n. 
In order to retard the motion of the dise B, and, at the 
