Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
N. 47, 48.] 
§18. A. Changes in the Retina due to Light 
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.


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