78 ON THE ROTATION OF COLOURED DISCS. 
same time, to produce the vibrations, the central aperture is 
made sufficiently large to admit of free motion on the 
spindle ; and there is appended, at or near its circumference, 
a light weight, such as a piece of string or silk, which, by its 
impulses on the atmosphere during rotation, both retards the 
motion and produces the vibrations. The proportion of the 
diameter of the central aperture of the disc B to that of the 
spindle of the colour-top is about as four to three—an 
aperture of four tenths of an inch, for example, to a spindle 
three tenths of an inch, is very effective. 
As in the kaleidoscope, so in this instrument, there is only 
one position for the situation of the eye with respect to the 
discs where the symmetry of the combinations is perfect, 
namely, vertically over the spindle of the top, so that the 
whole of the circular field can be distinctly seen. 
For the purpose of giving variety to the figures formed by 
the instrument, an assortment of fantastic patterns on. 
separate discs may be constructed to take the place of the 
disc 8; the disc a is also furnished with different colours. 
A disc of pure white forms exquisite gray combinations, with 
every conceivable variety of light and shade; a disc of white 
and green, in equal portions, is resolved into these elements, 
and their multiplication into a composite form is very agree- 
able to the eye. An elegant arrangement, whereby patterns 
may be coloured in the most attractive manner, is composed 
of half a disc of blue, and the rest of white, green, and red, 
in equal proportions.* 
The pictures thus presented to the eye are very beautiful. 
Their charm would appear to depend partly upon their being 
reflected to the eye through a perfectly black medium, 
which imparts brilliancy and illumination to the colours, and 
partly upon their being exhibited in a state of motion, the 
apparent and real direction of which bear no relation to one 
another. While the disc is actually performing one hundred 
revolutions per minute, and vibrating about thirty times 
during each revolution, the combinations themselves often 
appear hanging in space, trembling without progressing, or 
perfectly motionless, or gliding round in a direction contrary 
to that of their real motion. These frequently recurring 
and illusory changes excite the curiosity, give animation to 
the pictures, and confer an ethereal brightness, vivacity, and 
splendour upon them, which is altogether and peculiarly 
their own. 
* These coloured discs, as well as the designs already prepared on a 
black ground, may be procured of the publishers, Messrs. Sinith, Beck, and 
Beck. 
