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
The Sensations of Vision 
[332. N. 
a more delicate method, did succeed in detecting it to a slight extent. 
But even if it should be overwhelmingly proved that the photo-tropic 
reactions of the pigment epithelium do not occur in the human eye, 
that would be no argument against explaining the processes in the eyes 
of frogs, fishes, and birds in the above manner, and assuming that the 
same purpose, namely, adaptation of the eye to different intensities 
of light, is achieved in the eye of the mammal in a different way. The 
act of accommodation is performed by different classes of animals in 
fundamentally different ways. 
But there are other considerations against this hypothesis of an 
in-and-out mechanism. For instance it is not easy to see what is the 
purpose of the elongation of the cones in darkness. Why do not all of 
the cones remain immobile on the membrana limitans, as some of them 
do? What is the object of the pigment in the region of the human eye 
where there are no rods? How are we to explain the fact that over a 
considerable portion of the retina in the case of animals with a tapetum, 
like the dog and cat, there is no pigment at all? 
Without being able to answer such questions as these, it is im¬ 
possible to speak yet of a complete theory of the photo-mechanical 
processes in the retina. It is very probable that the process of light 
and dark adaptation is not rigidly connected with the existence of 
photo-tropic movements of retinal elements, but that these changes 
may accompany adaptation, the most essential basis of it being the 
formation and bleaching of visual purple. 
It is not probable that the pigment as such has anything to do with 
the formation of visual purple, because the latter occurs in large 
amounts even in albinos who have no pigment and in the parts of the 
retina of the cat where there is no pigment. The purpose of the pigment 
is probably purely optical and consists in the prevention of lateral 
diffusion of light in the layer of rods. But, as has been said, it is still 
obscure why with one set of animals this protective mechanism ex¬ 
hibits such plain reactions to light, and yet does not do so with other 


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