Harrisburg telegraph. [volume] (Harrisburg, Pa.) 1879-1948, 1919-04-02
= PERSISTENCE
OF VISION

VIII YOU plcuSe eNpldln WluC is
meant by persistence of Vision."-C.
D.
Your question comes just at the
moment when | have been reading
q description, by Professor Gaehr.
OF Wells College, of IA simple but
very effective device, whereby per-
sistence of vision may be plainly eX-
hibited to q large class or audience.
Moreover Professor Gaehr's de-
vice may furnish II valuable hint
to designers of stage illusions.
ghost scenes" etc, although it's
principle has already been en-
ployed for such purposes. Hero, Il
little condensed is Professor Gaehr'
own description.
Take a lantern slide bearing some
letters, and instead of imaging il
on q white surface, let the image
be absorbed by black velvet. or form
ed in an open doorway, which would
appear dark if the space behind
was unilluminated. Then move a
fairly white stick vertically down
the plane of the image. Different
portions of thei mage can then be
seen on the stick. and if the stick
moved fast enough the eye sees the
entire image easily.
1t is evident that here is the germ
of very startling stage effect, such
as the mysterious appearances OF
the warning sentence on the wall.
or in the air, at n representation of
"Belshazzar's Feast" But our con-
Germ with the device relates to the
light it throws on your question as
to the nature of persistence of vision.
If the stick is lIeld steadily in
one position only n small strip of
the image is rendered visible by
reflection from the white surface OF
the stick, all the rest of the image
remaining unseen because its light
is absorbed by the black back
ground. But when the stick is
moved. other parts of the image
successfully fall upon it, and if the
motion is rapid and of sufficient
range to cover the entire space oc-
cupied by the image, the whole OF
the latter will become visible
simultaneously. because the retina of
the eye has the property of retain
ing for a fraction of a second, any
light impression made upon it, and
if the rejecting stick moves fast
enough to pass from one side of
the image to the other before any
of the successive impressions have
had time to vanish, the whole will
necessarily be seen at one view.
The cause of the persistence of
the impression of the retina is
similar to that of the persistence
of any other physical impression,
being due, fundamentally, to the
principle of inertia. Anything that
is in motion can be stopped only by
force opposing the motion. This
applies to vibratory motion of nerve
cells and to all physical and chem
ical changes. Time is always re-
quired both to start and to arrest
any process. The nerve elements
of the retina continue in n state of
eXcitation for from 1-50th to 1-30th
of second.
Two successive impressions made
1-15th of n second apart may blend
so that tho eye sees them as one.
This is the principle on which the
cinematograph acts; the successive
photographs of a moving object arc
taken so rapidly that the images
blend together, and when the pic
ture is reproduced on the screen
the same effect of unbroken con-
tinuity is produced by running the
successive images at a proper speed
across the field of the lantern.
Thus what at first sight might
seem q defect of the human eye
turns out to be an advantage, for
if the retina did not retain the inn
pressions of light for n perceptible
time this world in which every
thing goes bp vibration would be a
jiggery place to live in.
There are q good many things
about the performance of our eyes
to give us interesting food for
thought. Take. for instance, the
fact, which seems so simple until
you begin to think about 1t, that
everything looks right side up to
us, although the image of every
thing is known to be wrong side
up on the retina. The rays of light
in passing through the lens of the
eye cross one another, so that
those coming from the top of a
tree, for example, fall at the bot
tom of the retina, while those COM
ing from the bottom fall at tho
top. Yet, in the brain, to which
the retina transmits it's impres.
sions, there is some power. or prop
erty which automatically reflects
the image. and we see Il upright.
The sense of vision in the brain
follows the lines of light back
ward, experiences no confusion
when they cross one another in
the eyeball, and projects the whole
image out of the eye back to the
object itself. and while so doing
turns it right side up. The image
is at the back of the eye. yet the
brain knows that the object which
the image stands for is entirely
outside the eye. And it knows
that although the only thing II
really sees Is the image, for it has
no means of seeing the actual
thing itself.
We can touch a thing directly
but we cannot see it directly. In
absolute darkness the sense of
touch still acts, but the sense OF
vision is gone. Mediate on these
things some restless night and II
they do not put you to sleep they
may make you wiser; at all events
in your own conceit.
REPUBLICANS WIN IN ST. LOUIS
st. Louis. April 2.-The entire Re-
publican ticket was victorious at yes
terday's municinal election aocord- =