The mirror. (Stillwater, Minn.) 1894-1925, 1904-12-08


re, must make the necessary allowance for
its distance from the succeeding tie
roughly estimated, all of which calls
for the expenditure of an excessive
amount of energy. When we walk
over a smooth and we'll known path
there is no stopping and starting at all,
but locomOtion is continuous and
comparatively easy. In such cases the
distance actually covered is judged ac-
cording to the amount of energy which
the limbs must put forth to cover the
distance. A similar illusion occurs
when the eye is called upon to survey
distances which-roughly speaking
correspond to the railroad ties and the
smooth path.
In exhibit two the extents indicated
by the letters, ga" and 3d." are equal,
sc" is an open space over which the
eye moves without restriction, while
sd" is series of ties and, altno the eye
to scan them with apparent ease,
there is in reality slight tendency to
take account of each cross tie, which
requires more energy than it does to
move the eye over an empty space of
like prOportion-hence it is that I'D" is
judged to be greater than "E," and "F"
is apparently shorter than &c" or
"H."
In connection with this illusion it
may be said that the effect depends
entirely on the relation of the number
of cross lines to the distance; with too
many or too few it becomes less notice.
BILE and after I had completed these
crude drawings discovered that had
failed to observe the correct proportion
while increasing the scale to accommo-
date an increased perspective.
Exhibit three shows two squares of
equal dimensions. The one labled "R"
is apparently much the larger and the
explanation is not dissimilar to that
prevailing in connection with the pre-
ceeding set of figures AS the eye
scans the checker board pattern there
is an unconscious tendency to take
account of each intersection and these
manifold stoppings aud startings cause
one to overestimate the size of the
square.
Exhibits two and three are but few
of the innumerable examples which
might be submitted to demonstrate
that occupied space is overestimated
and empty space underestimated. In
every such case the illusion results
from the one controlling cause. We
base our estimations of extents upon
the amount of eye movements which
is necessary to look over the field or
surface subjected to our inspection.
Without exception movements of the
normal eye are subject to the control
of three distinct sets of muscles which
are attached to and govern each eye.
While so adjusted that they can move
the eye in any desired direction these
muscles are not symmetrically placed,
and as a natural consequence it is
more difficult to move the eye IN Ger
tain directions than in others. A mo-
mentary experiment will disclose that
it is much easier to move the eyes from
right to left, and vice versa, than it is
to move them up and down. Such
being the case it logically follows that,
since we judge distances by the
amount of eye movement necessary
to cover them, there is a natural tend
ency to overestimate vertical dis
tances and to underestimate horizontal
distances.
Exhibit four shows two lines of equal
length even tho the vertical line looks
decidedly longer, and, fundamentally,
the explanation is to be found as above
noted. In this case, however, certain
conditions, purely local, serve to in
tensify the normal illusory effect.
irst-keeping in mind the primary
cause-we may note that the rectangu-
lar form of the cardboard contributes
to the effect and, again, recalling the
causes that governed in exhibit one, we
find that the series letter "E," placed
at the apex of the vertical line, invites
the gaze to wander beyond the termi-
nus of the line proper, while in con-
nection with the horizontal line the