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. 279, 280.j 
A. The Adaptation of the Eye 
angle of 8.6') to be about 1400 times more than the foveal sensitivity. 
Using mixed white light, subtending an angle of half a degree, after an 
hour’s adaptation, the writer found the sensitivity in the most sensitive 
part of the periphery to be in round numbers a thousand times greater 
than it was in the fovea under the same conditions. In this connection, 
it should be noted that the distribution of sensitivity in the light- 
adapted eye is entirely different, the maximum sensitivity to light in 
this case being in the fovea centralis. This is easy to prove by having 
a small point-source of light of intensity just a little above the foveal 
threshold somewhere in a dark room. If the eye is thoroughly light- 
adapted, it is very difficult to enter the room and find this spot of light 
right away. It is almost by accident that it is seen, and then it is 
surprisingly distinct, although it may be quite easily lost again. On 
the other hand, a dark-adapted eye will have no difficulty in discover¬ 
ing the luminous spot immediately, because the peripheral sensitivity 
is greater than that in the fovea. 
Accurate measurements of the decrease of sensitivity from the fovea 
to the periphery of the light-adapted eye, with due consideration for 
the state of adaptation, cannot be made except with coloured lights. 
However, in the observations on the distribution of sensitivity to 
coloured lights in the light-adapted eye, as carried out at the writer’s 
suggestion by C. L. Vaughan and A. Boltunow1, the decrease of 
sensitivity out towards the periphery was found to be exactly of the 
same nature for red, green, and blue lights; and, perhaps, therefore, 
the quantitative results of these observers may be applied without 
hesitation to all light of any kind at all. What they found was that 
the sensitivity 10° to one side of the centre of the fovea was not more 
than one-fourth of its value in the fovea; at 20° away it was one-tenth; 
and at 35° away one-fortieth. 
The relative central scotoma of the dark-adapted eye. As has been 
already stated, light stimuli, whose intensities are below the threshold 
value of the fovea cannot be seen in a central region of about two de¬ 
grees, but are visible immediately as soon as they fall on the retina 
outside this area after it has been made more sensitive by being dark- 
adapted. The region within which stimuli of still lower intensity 
cannot be perceived is correspondingly wider. The dimensions of this 
“blind region” are given in the fourth column in Table IV for the 
intensity of light employed in that case. 
1 Vaughan u. Boltunow, Über die Verteilung der Empfindlichkeit für farbige Lichter 
im helladaptierten Auge. Zeitschr. f. Sinnesphysiol. NLII. 1. 1907. See also Guillery, 
Zeitschr. f. Psychol, u. Physiol, d. Sinnesorg. XII. 261. 1896; XIII. 189. 1897.


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