Volltext: Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition (2)

The Sensations of Vision 
[315, 316. N. 
white, but we must assume that the operation of the mechanism of 
vision depends on other factors besides. Schaternikoff found that 
the fusion frequencies of the photopic eye and the scotopic eye were 
in the ratio of 5 to 3. For as exact a match as possible, as to both 
brightness and colour, the writer has obtained bigger differences, 
with his eyes in the two states of adaptation. Discs like those described 
by Helmholtz in connection with Fig. 42 are convenient for observa¬ 
tions of this kind. Experimenting with this apparatus, with one eye 
thoroughly dark-adapted and the other light-adapted, the writer found 
that the speed of rotation could be so regulated that for the photopic 
eye there is no flicker at all in the outside ring, little in the middle, and 
distinct flicker in the central ring; whereas the entire disc was free 
from flicker so far as the scotopic eye was concerned. The brightness 
was the same under the two conditions. 
Without paying special attention to the state of adaptation, 
Porter1 compared the fusion frequencies for a series of different 
intensities of illumination from the highest to the lowest, and obtained 
the remarkable result that the relation between the frequency and the 
intensity of illumination can be expressed by a curve composed of two 
approximately straight portions meeting each other at an angle, similar 
to the curve in Fig. 70 which represents König’s measurements of visual 
acuity. In each of the straight portions the fusion frequency increases 
in proportion to the logarithm of the intensity, the factor of pro¬ 
portionality, however, being different for the two branches. 
The similarity between the two curves, as v. Kries2 has pointed 
out, becomes still more perfect when the intensity is noted at which 
the bend in the curve occurs. This intensity, at which there is suddenly 
a new relation between intensity of light and spatial and temporal 
power of discrimination, turns out to be in fact practically the same 
in both cases, being the illumination of a Hefner lamp at a distance 
of one-tenth of a metre in König’s experiments and that of a standard 
candle at a distance of one-eighth of a metre in Porter’s experiments.3 
Aside from the fusion of periodic luminous stimuli, the time rela¬ 
tions in a single brief stimulation are likewise of interest in connection 
with the duplicity theory. These quite complicated phenomena are to 
be described hereafter. It will appear then that although it is not yet 
possible to give a complete explanation of them, still they present a 
number of characteristics indicating with much probability a difference 
1 Proceedings of the Royal Society, London, LXX. 313. 
2 J. v. Kries, Über die Wahrnehmung des Flimmerns durch normale und durch total 
farbenblinde Personen. Zeitschr. f. Psych, u. Physiol, d. Sinnesorg. XXXII. 113; and 
Abhandlungen zur Physiol, d. Gesichtsempfindungen. Drittes Heft 1908. 
\1fSee H. D. Parsons, loc. dt., p. 208. (J. P. C. S.)


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