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616 BRAIN AND NERVES.
absent, and is also necessarily absent in the macula lutea. In a variety of bat
(Rhinolophus hipposideros), in hens, pigeons and new-born rabbits, no visual
purple has been found in the rods.
A solution of visual purple in water which contains 2-5 per cent crys-
tallized bile, which is the best solvent for it, is purple-red in color, quite
clear, and not fluorescent. On evaporating this solution in vacuo we
obtain a residue similar to ammonium carminate which contains violet
or black grains. If the above solution is dialyzed with water, the bile
diffuses and the visual purple separates as a violet mass. Under all
circumstances, even when still in the retina, the visual purple is quickly
bleached by direct sunlight, and with diffused light with a rapidity cor-
responding to the intensity of the light. It passes from red and orange
to yellow. Red light bleaches the visual purple slowly; the ultra-red
light does not bleach it at all. A solution of visual purple shows no special
absorption bands, but only a general absorption which extends from the
red side, beginning at D and extending to the G line. The strongest
absorption is found at E.
Koettgen and Abelsdorf" r
have shown that there are, in accordance with
Kuhne’s views, two varieties of visual purple, the one occurring in mammals,
birds, and amphibians, and the other, which is more violet-red, in fishes. The
first has its maximum absorption in the green and the other in the yellowish-
green.
Visual purple when heated to 52-53° C. is destroyed after several
hours, and almost instantly when heated to 76° C. It is also destroyed
by alkalies, acids, alcohol, ether, and chloroform. On the contrary,
it resists the action of ammonia or alum solution.
As the visual purple is easily destroyed by light, it must therefore also be
regenerated during life. Kuhne has also found that the retina of the eye of the
frog becomes bleached when exposed for a long time to strong sunlight, and that
its color gradually returns when the animal is placed in the dark. This regenera-
tion of the visual purple is a function of the living cells in the layer of the pigment
epithelium of the retina. This may be inferred from the fact that a detached
piece of the retina which has been bleached by light may have its visual purple
restored if it is carefully laid on the choroid having layers of the pigment-epithe-
lium attached. The regeneration has, it seems, nothing to do with the dark
pigment, the melanin or fuscin, in the epithelium cells. A partial regeneration
seems, according to Kuhne, to be possible in the retina which has been completely
removed. On account of this property of the visual purple of being bleached
by light during life we may, as Kuhne has shown, under special conditions and
by observing special precautions, obtain after death, by the action of intense
light or more continuous light, the picture of bright objects, such as windows
and the like—so-called optograms.
The physiological importance of visual purple is unknown. It follows
that the visual purple is not essential to sight, since it is absent in certain
animals and also in the cones.
1
Centralbl. f. Physiol., 9; also Maly’s Jahresber., 25, 351.
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