Bauhaus-Universität Weimar

Dictionary of philosophy and psychology including many of the principal conceptions of ethics, logics, aesthetics ... and giving a terminology in English, French, German and Italian, vol. 1 [a-laws]
Baldwin, James Mark
those viewed in direct vision, ceteris paribus, 
owing probably to the tendency of the eye to 
follow the object in the latter case. If the eye he 
fixed on a point at rest, the latter after a time 
will appear to move about slightly ; the motion 
is especially marked when the object fixated 
is the only clearly defined point in the field 
of vision ; it is really due to involuntary eye 
movements. Exner terms the experience 
‘ autokinetic sensations.’ If the eye-ball be 
pressed from side to side with the finger, the 
objects in the field of vision appear to move; 
but no such interpretation occurs when the 
eye moves normally. 
A somewhat different instance of relativity 
is observed in the apparatus called anortho- 
scope. This consists of a paper with a narrow 
vertical slit, which is held before one eye, and 
a circle slightly less in diameter than the 
length of the slit ; when this circle is moved 
rapidly from side to side behind the slit, it 
appears as an ellipse, the horizontal axis 
becoming shorter as the rate is increased; 
when the motion is slow, the illusion dis¬ 
appears, or is reversed, the horizontal axis 
becoming longer than the vertical. 
Illusions of the direction of motion may be 
due to a false judgment of perspective, or to 
the undue prominence of certain factors and 
overlooking of others. An example of the 
former appears when we observe the motion 
of a windmill from an oblique angle at a 
distance ; either side of the mill may be made 
to appear the nearer, so that the upper vanes 
seem to move now towards, now away from us. 
A similar illusion of perspective may be 
observed with vessels on the water. An 
illusion of direction due to the obscurity of 
certain factors is observed when a spiral 
figure is rotated rapidly about its centre on 
a colour-wheel ; the rotary motion is indis¬ 
tinguishable, and there appears instead a 
motion of all points of the spiral towards 
the centre, or away from it, according to the 
direction of rotation. If a card with a 
number of broad concentric circles like a 
target be moved in a circle before the eyes 
without rotation of the card, the figure becomes 
partly blurred, and the blurred sectors appear 
to rotate in the direction of the actual move¬ 
ment. If a figure consisting of a circle with 
teeth projecting inwards or outwards (like cogs) 
be moved in a similar manner, the cogs will 
appear to rotate around the circumference of 
the circle. Figures of this sort are known 
as strobic circles. 
The illusion of rest is illustrated in a 
rapidly rotating wheel, whose spokes present 
the appearance of a continuous, motionless, 
and semi-transparent surface. In the thauma- 
trope different pictures or letters are printed 
on the two sides of a card, which is rotated 
by twirling two strings attached to the right 
and left edges; the two pictures combine 
into one, or the letters into words, which 
appear to remain stationary before the eyes. 
An opposite illusion appears in the strobo¬ 
scope. This consists of a circular card with 
a number of axial slits, and another circular 
card, placed behind it, with a series of dots 
or figures placed around the circumference, 
representing different phases of an object in 
motion. The two cards may be rotated in the 
same or in opposite directions ; looking 
through the slits as they pass, we catch 
momentary glimpses of the successive images, 
and when the rotation is rapid they appear as 
a single object having a continuous motion. 
The figures may be placed on the back of the 
first card, and reflected by a mirror, which is 
substituted for the second card. There are 
various forms of stroboscope, called dedalium, 
phenakistoscope, zoetrope, &c. In the zoetrope 
the slits are cut in the side of a short cylinder, 
like the cover of a bandbox, while the figures 
are on a strip, which is placed upright around 
the inside of a cylinder; with a series of 
instantaneous photographs a very realistic 
appearance of continuous motion is obtained. 
In these forms of stroboscope the pictures 
must be the same distance apart as the slits. 
Recent applications of the stroboscopic illusion 
are found in the kinetoscope, mutoscope, vita- 
scope, cinematograph, biograph, &c. Here 
the pictures follow one another in rapid 
succession, remaining in view for a period 
which is much longer than the time occupied 
by the transition ; the latter is so short as to 
make the real motion imperceptible, while the 
pictures at rest are clearly perceived ; this 
avoids the necessity of slits. The pictures 
represent very near phases of motion, and the 
result is an illusion of motion which is perfect 
in proportion to the degree of perfection of 
the apparatus ; the rate of the illusory motion 
may be made the same as in life. The 
pictures may be enlarged and projected upon 
a screen by means of a lantern. 
With certain coloured diagrams a con¬ 
tinuous to-and-fro motion gives rise to the 
appearance of a sudden springing of the 
figures from side to side ; this illusion is 
known as the ‘ fluttering heart,’ or chromato- 
L 1 


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