The Republican. [volume] (Oakland, Md.) 1877-current, 1877-04-14

Illuminated by yellow light. The eve
ball was opened and instantly plunged
INTO per cent. solution of alum. This
occupied two minutes, and the other eye,
still remaining in the head, was then eX-
posed at the window justa as the first had
been. 1t was then taken out and placed
In the alum solution like its fellow. The
next morning the two retin9 were care
1ully isolated, separated from the optic
nerve, and turned On a beautiful rose
red ground a sharp image, somewhat
more than one miliimetre (one-twenty-
NTH lnch) square was found. The image
on the first retina-that which was eX-
posed during life-was somewhat red
dish and not so sharply defined as that
on the other.
This tixature of the last impression on
the living retina is by no means an acct
dental discovery, but is the final step in
laborious series of delicate researches.
Nor is it the triumph of one man alone,
the preliminary work having been per-
formed by two distinguished physiolo-
gists. Prof. Boil, of Rome, discovered
that the external layer of the retina in
all living animals has purple color,
which is destroyed by light During life
the color is perpetually restored by dark
ness, but after death, Boil thought, it
disappeared entirely. Prof. Kuhne fol-
lowed up this wonderful discovery, and
confirmed it in general, while correcting
some of Bolts conclusions. He first as
certained that death does not necessarily
DESTROY the color, since a retiEa that is
not exposed to white light, but is kept in
room lighted by a yellow sodium flame,
retains this "vision purple" for twenty
four or twenty eight hours, even though
incipient decomposition may have set in.
1t is destroyed at the temperature of
boiling water or by immersion in alcohol,
glacial acetic acid, and strong solution
OF soda, but in strong ammonia, saturated
solution of common salt or glycerine, it
remains undiminished for twentv-four
hours. On testing the effect Of different
colored lights upon this ''vision purple, ="
he found that the most refrangible RAYS
changed it most, while red has hardly
more effect than yellow light The color
is not 50 delicate as Boil supposed. A
few moments exposure to daylight does
not bleach the retina. This requires OK
posure for a considerable time to direct
sunlight. The source of the color was
found to be the inner surface of the
choroid, upon which the retina lies. If
a portion of the retina is disengaged
from the choroid and raised up, it
bleaches, though the remainder, still at
tached portion, retains its color. If the
raised flap is carefully replaced upon the
choroid 1t regains its purple hue. This
restoration is believed to be a function
Of the living choroid, and probably of
the retinal epithelium, though it is indie
pendent of the black pigment which this
epithelium contains. This vision purple
is the latest discovery in optical physi-
olo9y, and it cannot fail to be a most i'm
portant one. How far it will alter the
received views upon the subject of
changes in the strength of vision, which
are now attributed to alterations in the
distance of the crystalline lens, cannot
be foretold. But it may be found posi-
BILE to stimulate by drugs the restorative
| action of the choroid, and thus, by gain
ing increased definition, improve weak
sight. AS to the detection of murderers
by photographing the last retinal pict-
ures from their victims eyes, while these
discoveries do not leave this an impossi-
bility, they do not much improve the
probability of it ever being done. Very
often the sight of the assassin is not the
last which comes within the victims
vision. Too long time also usually
elapses before discovery. These and
similar difficulties must prevent the
utilization Of these discoveries in this
direction, even if they should prove to
be in themselves all that is hoped. The
retinal picture has not yet been photo,
graphed, but it seems probable, from the
above-recOunted experiments, that it can
be.-Galary.