There are several ways of illustrating the retention by the retina of
the eye of the images of objects after they have been withdrawn from
sight, but none is so curious as the philosophical toy called the Magic
Disc, which, from the optical principles involved in its extraordinary
effects, deserves to be noticed as one of the remarkable inventions of
the present century.
One of the most striking methods of exhibiting the retentive property
of the retina, before the invention of the Magic Disc, was to paint
different objects at the back and on the front of a card, and by then
giving rapid rotation to the card, both objects were seen together.
Thus, when the figure of a bird is painted on one side, and an empty
cage on the other, by rapidly turning the card, the bird appears to be
in the cage. In the Magic Disc the objects are painted on the same side
of a circular piece of card-board, and both are exposed to view during
their rapid rotation.
The disc is divided into eight or ten compartments, in each one of
which the same figures are repeated, though the positions of one
or more of them are changed. A favourite subject represented is a
clown leaping over the back of a pantaloon, which affords a simple
illustration of the apparent relative movements of two bodies, and will
serve to explain how the effect is produced.
[Illustration]
The instrument consists of a disc of stiff card-board, about nine
inches diameter, mounted on a horizontal pivot in the centre, on which
it may be freely turned. Between each of the compartments of the disc
there is an elongated aperture, about one inch long and a quarter of an
inch wide, for the eye to look through. Suppose the disc to be divided
into eight compartments, by radial lines. In the compartment No. 1,
the pantaloon is represented in a stooping posture, and the clown is
on the ground ready to make a spring. In No. 2 the pantaloon is in the
same attitude, but the clown has commenced his leap, and is raised a
little way from the ground. In the third division he is shown still
higher in the air; and in the fourth he is mounted above the shoulders
of pantaloon, who retains the same posture as at first. The fifth
compartment represents the clown as having jumped over pantaloon's
head, and coming down to the ground; and in each succeeding division
his farther descent is shown, till, in No. 8, he has reached the ground
again, and is ready to recommence the leap.
When the disc is turned rapidly round on its pivot, the figures
painted upon it are mingled together, and present a confused medley of
lines and colours, in which no object can be distinctly defined. This
mingling of the objects is caused by the retention of the images by the
retina, so that if the eye be directed to any point, the impression of
the lines and colours that pass rapidly before it is not effaced before
another and another appear to produce fresh impressions, and they
mingle together in confusion. If, for instance, there were a circle
formed of dots marked on the disc, the impression of each dot on the
retina would be prolonged; and as, by the rotation, other dots would
come into the field of view before the impression of the first was
removed, it would form an unbroken ring. But if the disc were screened
from sight, at intervals of nearly equal duration to that of the
continuous impression, so as to efface the image of one dot before the
rays of another were admitted to the eye, then the ring would be seen
to be composed of dots, as distinctly as when the disc was stationary.
The effect of screening the objects from the eye at short intervals
is produced by looking with one eye through the openings at the image
of the disc, reflected from a mirror. The figures are then seen only
when the apertures come opposite the eye; but as the impression of one
view remains till it is renewed by the light admitted through the next
aperture, there is continuous vision of the objects painted on the disc.
It is thus that the figures of pantaloon and clown become visible, and
their apparent relative movements are occasioned. For instance; each
time that the impression of the figure of the pantaloon is renewed,
he is seen in the same place and in the same attitude; therefore he
appears to be stationary, though the successive pictures that compose
his figure to the eye are in rapid rotary motion. The figure of the
clown, however, is seen in a different position each time that he
comes into view, therefore he appears to be in motion relatively to
pantaloon, though stationary as regards his absolute position on the
disc.
The same effect would be produced if the disc, during its rotation,
were seen by successive electric sparks. The electric spark is so
momentary in its duration, that the most rapidly moving objects appear
stationary; therefore each spark would show a seemingly stationary
disc, on which the figure of the clown would appear in different
relative positions; and the illusion would be as perfect as when the
rays of light are interrupted at intervals.
The electric spark is so instantaneous that a cannon ball might be
seen in its rapid flight, if illuminated by a flash of lightning, and
would seem to be stationary. Professor Faraday mentioned, in one of his
lectures, the extraordinary appearance which a man, who was jumping
over a stile, presented when seen by lightning on a dark night. The man
seemed to be resting horizontally in the air, with one hand touching
the stile.
The duration of the impression of an object on the retina is capable of
illustration by means of the Magic Disc in a great variety of designs,
each one of which may represent many movements. The turning of the
wheels of machinery, the tossing of balls, the dancing figures of men
and women may thus be shown, the designs for which afford ample scope
for exercising the pencil of an ingenious artist.