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. 327, 328.] 
B. Duplicity Theory and Twilight Vision 
The close connection between the optical properties of visual 
purple and the stimulating effect of spectral lights, which is proved to 
exist by the researches alluded to above, comes out still more clearly 
in a series of experiments carried out by Stegmann under v. Kries’s 
direction. Visual purple, as is well known, occurs in the outer segments 
of the rods, colouring them in their entire length, and hence it must 
be in a layer of some little thickness. Light which traverses the layer 
with its purple contents will undergo a partial absorption in the layers 
which it encounters first, and in fact those rays will be most absorbed, 
and consequently most enfeebled, that have the strongest stimulating 
actions on the rods, namely, the green rays. The more concentrated 
the “solution” of visual purple is in the layer of rods, the greater the 
absorption will be. Thus, the ratio between the stimulating effects 
of two kinds of light, one of which (say, green) was strongly absorbed, 
while the other (say, orange) was not much absorbed, would have to 
depend to a certain extent on the amount (or “concentration”) of 
visual purple in the layer of rods. Now we know it to be a fact that 
the acumulation of visual purple in the retina is notably increased by 
long-continued exclusion of light; and therefore matches made under 
the conditions of twilight vision between two kinds of light like those 
mentioned above will generally not be independent of the duration of 
the previous dark adaptation. Asa matter of fact, Stegmann and the 
writer, working together, found that there was no doubt about it. 
If, for example, after being dark-adapted for a period of five or ten 
minutes, “twilight matches” were made between spectral orange and 
blue-green (both of which would, of course, look colourless), and if 
then this match was considered by the same eye after it had been 
dark-adapted for a much longer time, it ceased to be valid: the light 
of longer wave-length (orange) looked the brighter of the two, and it 
was necessary to reduce its intensity to three-fourths of what it had 
been. Accordingly, this is a change in the opposite sense from that 
which would correspond to the Purkinje phenomenon; but it occurs 
in the direction that was to be expected if the increased accumulation 
of visual purple has any influence on the stimulating effect. The 
observation also shows that the place of action of light in the rods 
cannot be, or at least cannot be entirely, in the inner segment, or at 
the border between the inner and outer segments, but must be farther 
outwards. The farther outwards in the rod the place of stimulation 
is, the more the absorptive action of the visual purple must influence 
the threshold value. On the other hand, it does not follow from the 
experiment just described that the place of stimulation is only at the 
outermost end of the rod. It may just as well take place in the entire 
outer segment.


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