N. 47, 48.]
§18. A. Changes in the Retina due to Light
55
Wave-length 589 542 530 519 509 491 474 459
Bleaching value 1 3.40 3.62 3.45 3.09 1.69 0.975 0.299
Both in the live eye and also, under some circumstances, in the
enucleated eye, there is a regeneration of visual purple after bleaching;
and to a certain extent even in isolated retinas and solutions. When
both eyes of a live frog have been exposed to sunlight for half an hour,
and the animal has then been killed and the eyeballs taken out, the
retina of the eye that is opened immediately will be found to be
without colour ; but if the other eye has been kept an hour in the dark
in a damp receptacle, the retina will be a purple-red. In the case of
the frog, Kühne detected the first trace of red after complete bleaching
twenty minutes after shutting off the light; whereas in the case of the
rabbit there were signs of this colour in about five minutes. The re¬
generation is by far the best and most complete when the retina is in
contact with the pigment epithelium. A retina taken from an eye that
is without pigment never regains the perfect red colour.
According to Kühne and Garten, the most favourable condition
for the regeneration of the purple in the isolated retina was when the
bleaching had been permitted to proceed as far as the yellow, the retina
then being placed in the dark. Apparently, therefore, the visual purple
is most easily produced anew from the products of its own decomposi¬
tion before they have lost all colour. When the retina has been
bleached completely, regeneration does not pass through all the inter¬
mediate stages of yellow, orange and red, but the retina becomes
bright lilac, and then pink. In this case, therefore, the process of
formation of the purple must be different from that when the purple is
recovered from the yellowish product.
Both bleaching and regeneration depend on the temperature.1
Regeneration, in particular, is much retarded by cold; for example, the
retina of a frog at 0° C takes nine hours to regain its purple colour.
In warm-blooded animals the regenerative ability is lost a few minutes
after death or after circulation ceases. Evidently, the damage is
greatest in this case to the pigment epithelium which is so important
for regeneration. Whatever our knowledge may be as to the physiology
of visual purple in solution and in the isolated retina, it is doubtful
how far it can be applied in the case of the eye of a warm-blooded
animal with circulation intact.
The fluorescence of the retina2 when radiated by ultra-violet
light is another remarkable property. It is much more pronounced in
1 ^[See Hecht, loc. cit. (H. L.)
2 Helmholtz, Pogg. Ann. XCIV (1855); Setschenow, v. Graefes Arch.f. Ophthalm.
V. 1859.