OPTICS. 127 
disks. If, in the first mentioned case, the left eye be closed and the left disk 
steadily looked at, the right disk will completely disappear, on account of 
the falling of its image on the punctum cecum. The experiment, to be 
successful, may, for particular individuals, require a variation in the vertical 
height of the eye, as also in the distance between the disks. 
The impression of an object upon any point of the retina lasts for an 
appreciable length of time after the object has been withdrawn. For this 
reason, a burning coal, swung quickly round, exhibits the appearance of a 
luminous circle. A circle (fig. 87) whose sectors are alternately white and 
black, will, when rotated rapidly, exhibit a grey color. If, on one side of a 
circular disk, a horse be painted, and on the other a rider, and the disk be 
rotated rapidly on the transverse diameter as an axis, the rider will appear 
to be seated on the horse. 
The motion of an object may be sometimes so rapid as to produce no 
impression on the retina. Thus, in the case of a wheel in very rapid motion, 
the spokes will disappear entirely, leaving nothing visible but the circum- 
ference and the centre. 
The Phenakistoscope ( fig. 55) is an ingenious apparatus constructed to 
illustrate the principle of the duration of the impression of light on the 
retina. This isa disk of six to nine inches in diameter, which can be 
turned rapidly about a horizontal axis, and near whose edge there are 
several holes (eight in our figure) at equal distances apart. Inside of these 
apertures is attached a smaller painted disk, on which one and the same 
object is painted in various successive positions, each hole corresponding to 
one of these. Our figure represents a pendulum in its various positions. 
The apparatus is now to be held before a mirror, with the painted side 
towards it, so that the image may be seen through one of the openings, the 
upper for instance. By revolving the disk rapidly, the optical impression 
produced will be that of an oscillating pendulum. Other objects besides a 
pendulum may be used, and the movements of men and other objects may 
be simulated with the most remarkable success, by a proper arrangement 
of the various positions. Farapay has examined these appearances with 
great care, and tried many experiments on the subject.. He found that 
when the number of the images is less than that of the holes through which 
they are observed, the images appeared to change their place, and go 
backwards. The contrary was the result when the number of images 
exceeded that of the apertures. 
Impressions of colors which do not exist in surrounding objects are often 
experienced by the retina. Such colors are called subjective. Here belong 
the so-called after-images. If we gaze intently at the flame of a candle, 
and then closing the eye. direct it towards the dark side of the room, the 
flame will appear to be distinctly visible, becoming in succession, yellow, 
orange, red, violet and greenish blue, finally vanishing entirely. If the eye 
be directed towards the bright side of the room, the colors will be presented 
in an inverse order. Again, if we look at the dark frames of a window, 
relieved against a clear sky, and then closing the eye, direct it towards 
a white wall, we shall see a light frame with intervening dark spaces. 
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