The Free lance., February 01, 1890, Image 11

mind, because it cannot think two objects
there, and says there is but one.
If the axes of the eyes be so directed as to
meet a few inches beyond some objects, as
at a lamp flame beyond one's finger, then each
eye sees an image in a different direction,
and the mind, having no difficulty in con
ceiving two things exactly alike, only so
they be not in the same place, says there
are two, or we see two things, while there
is but one.
Again, let the axes of the eyes be con
veyed so as to cross a few inches in front of
two similar pictures (which must be small
enough that the corresponding points on
them are nearer to the centres of the pupils) :
then the image of the left object in the
right eye and that of the right object in the
left eye lie upon each other, in that nearer
plane passing through the intersection of
the axes, and the mind says there is but
one object. There are two complete and
perfect affections in visual sense, each cans
ing perfect perception ; but both perceptions
pertaining to the same position in space, and
because two things cannot be in the same
place at the same time, the mind, notwith
standing the doubleness of perception, forms
but one conception. We see but one, when
there are really two. In the first instant
of this experiment there are four pictures
seen, of which the two middle ones are in an
instant brought together so as to seem but
one. If now the two side images be cut
off by a piece of paste-board, properly
placed and having a hole of right size in
it, a stereoscope is formed, all but the mag
nifying power : but it has this remarkable
peculiarity, that it reverses everything. It
may be called a stereoscope of reversion,
and affords much interest.
Once more, change the direction of the
optic axes so that they shall meet a few
inches farther away than two similar objects
or pictures. Take two lighted candles and
THE FREE LANCE.
read a paper held a few inches behind them.
So long as the candles are further apart than
the eyes, no special phenomenon is observa
ble ; but when nearer together than the eyes,
a third and brighter flame appears mid-way
between them. This image lies beyond the
lights at the point where the line from 'the
left eye to the left object (flame), meets the
line from the right eye to the right object ;
where there is projected an image of each of
the flames. But as the mind cannot think
two things simultaneously in the place it says
there is but one—and so we see but one. NoW
if the two side images be cut off by a card
board, held vertically between the eyes so
that the left eye cannot see the right object
nor the right eye the left, only the middle
image will remain ; which is the one seen
in the common stereoscope, only it is with
out lenses. What then is the use of lenses,
if all the appearance of solidity is so easily
obtained without them ? The first use is
to enable us to use larger pictures, whose
corresponding points are farther apart than
the pupil centres of the eyes and the
second use is to magnify these larger pict
ures so that they shall appear still larger
and in better detail.
The very common idea that the appearance
of roundness and solidity, which is the
wonder of the stereoscope, is due to the
lenses is an entire mistake. To what, then,
is this marvelous effect due ? To answer
this we must first answer two other ques
tions. First, when does the mind conceive
a thing to be a solid rather than a surface ?
And this involves the second, how do we see
distance ? The mind conceives a thing to
be a solid when there is perception of the
three dimensions of space. When we look
,at any object, we' perceive length and width
directly ; but we get no idea of a solid unless
we see a third;dimension, distance, or some
part further off than others. There are two
ways of perceiving distance. First, by appar