Year: 1872

after they are out of sight; or, to state
| It'll less familiar way, the impression


made by light on the retina of the
eyes does not cease when the light IL


removed, but continues for about the
eighth of a second after. That is a


very brief time, to be sure, but is long


enough to give rise to many peculiar
effects. We all know that if stick


with spark of fire ON the enl of it be


whirled round rapidly, we zee q circle


of light. The end of the stick appears


to be at every point of the circle at


once, which Of course, it cannot be.
"Seeing is believing" we say but in


this case, as in many others, the


ancient "saw" fails US. The luminous


impressions made by the spark at diff


ferent points, succeed one another SO


rapidly that they appear continuous.


We know also that when q wheel is
the spokes. In this instance the sue
the same part of the retinia, obliterate
one another, so that instead Of seeing
more than actually exists las the
lighted stick. We see less than we are
looking at. 1t will seem to be another
paradox when you assert that ii we


would look at the whirliDg wheel for
only the thousandth part Of q second
instead of a whole second or more, we
should see every spoke as distinctly ss
if the wheel were at rest.


Many optical toys have been made
which depend upon this law of vision.
The thaunadtrope is one of the simplest
of them. Two pictures are put on op-
posite sides of a circular piece of card,
which is rapidly by means of strings
fastened to opposite points of its edge.
The two images combine in the eye,
and we see one picture made up of the
two. A bird may be put on one side
of the card, and q cage on the other,
and we see the bird in the cage when
the card Is twirled. So a man on one
side and a horse on the other may
unite to form an EQUESTRIAN groupe.
You can get an idea of an effect by
making distinct horizontal line with
ink on one side of a card, and an up
right line on the other, which will
form cross if you twirl the card.


The zoetrope is another ingenious
and amusing contrivance, the effects
of which are due to the same cause. 1t
is cylindrical box, open at the top,
turning upon an upright axis. Around
the upper part of q box is row of
vertical slits. The successive positions
which moving 6ody assumes are rep
resented in order upon strip of paper;
and this paper is put within q box,
which is then whirled rapidly. If we
look through the slits, the pictures
come before the eye one after another,
and the impression of each lasts till
the next arrives, 80 that they all blend
together and the object appears to be
actually going through the motions de-
picted.


The Intermittent view of the pictures
got by looking through the narrow
Sits is essential to the effect. If we
look in at the top to see the pictures
all at once as they are turned round,
they will blend undistinguisIably, like
the spOkes of the wheel Ia rapid rOta-
ton. The slits, by interrupting the
view, limit the time that each is visible,
and allow an interval for the percep-
ton of one impression before another
is made upon the retina. The adjust
ment of the number and breadth of
the Sits, so that the successive i'm
pressions may blend in such a way,
that the motions may appear natural
and at the same time be clearly seen,
is matter of considerable nicely.


We have said above that the view of
a swiftly rotating wheel could be linn
tied, to very small fraction of sec
ond, we should see it as distinctly as
if it were motiOnless. We can accom-
plish this by lighting up the wheel
by an electric spark in dark room.
Wheatstone has proved that the dura-
tiOn of the spark is not more than 1-42,-
600 of a second; and viewed under
this brief illumination, wheel ro-
tating at the rate of a hundred times
a second appears to be absolutely at
rest. A single picture of it is formed
in the eye, and the light is gone before
a second impression is made upon the
retina. For the same reason, objects
made visible by a flash of lightning ap-
pear stationary, no matter how fast
they may be moving.


The fact that we do not cease to see
while winking is of course explained
by this principle of persistence of vision.
The eyes are closed for less than the
eighth of a second, and the impression
upon the retina is renewed Before it
has time to fade away.-JouymaI Of
Chemistry.