Illusions of the Eye are numberless, and afford a wide field for
experiment. Some people are left-eyed, others right-eyed, and very
few use both eyes equally. It is impossible to tell how far they
really do deceive us unless they have been tested in the proper
manner. For instance, if you ask anyone to what height a bell-topper
would reach if placed on the floor against the wall, nine times out
of ten the height guessed will be half as much again as the real
height of the hat. Everyone seems to _over_-estimate the proper
height.
[Illustration]
Another favourite illusion is to ask a person to mark on the wall a
height from the floor which would represent the length of a horse’s
head: here the majority guess far too little--for a horse’s head is
much longer than most people imagine, ranging from 25 to 34 inches.
In a recent experiment 5 persons out of 6 _under_-estimated the
proper height.
[Illustration]
Here are two triangles. Which is the one whose centre is the better
indicated? (It looks like A, but it is B).
[Illustration]
Again: Out of the two straight lines C and D which is the longer? (By
measurement we see they are both the same).
[Illustration]
Guess, by eye-measurement only, the longest and shortest of the three
lines marked A A, B B, and C C. When you have done guessing measure,
and see how much you are out.
[Illustration]
Which is the tallest gentleman of the three appearing in adjoining
figure?--Many would imagine the last to be the tallest, and the
first the shortest, whereas the reverse is the case--the last is the
shortest, and the first the tallest.
[Illustration]
It is surprising how the eye can be deceived, when dealing with areas
or circles. Place on the table a half-crown and a threepenny-piece;
let these be, say, 9 or 10 inches apart, and ask a friend how many
of the latter can be placed on the former--with this proviso: the
threepenny-pieces must not rest on each other, nor must they overlap
the outer rim of the half-crown; they must be fairly within the
circumference of the larger coin. Many will answer 6, 5, or 4, others
who are more cautious 3. Try for yourself and see how many you can
put on, and you are sure to be surprised.