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. 297, 298.] 
B. Duplicity Theory and Twilight Vision 
The statement that this is a typical curve means that it not only 
represents the case of persons with the same kind of colour vision, but 
cases of so-called normal colour vision and of various congenital types 
of colour blindness as well. It applies likewise, as has been shown by 
the writer, to anomalous trichromats to be described later. 
In his experiments on the luminosities of the spectral colours for various 
absolute intensities, A. König (Ges. Abhandl. p. 144, and Beiträge zur 
Psychologie und Physiologie der Sinnesorgane, Festschrift zu Helmholtz 70 
Geburtstag. 1891. p. 309) used for the lowest degree of intensity one that”was 
probably not far from the conditions required for pure twilight vision. On 
this occasion he got the important result that the great differences in the dis¬ 
tribution of spectral brightness, that exist between normal, red-blind, green- 
blind and totally colour-blind persons when the intensity of the light used is 
relatively high, disappeared almost entirely when the lowest degree of inten¬ 
sity was employed. More recent investigations have completely corroborated 
this result and shown that it is valid also for the anomalous trichromats, as 
above stated. 
21 2Z 23 Z%7 2627 28 23 30 31 
o> ©> cS ^ CTpïôïGî 
Fig. 67.—Distribution of the twilight values in the dispersion spectrum of gaslight 
(according to Nagel). The crosses indicate the results of previous, perhaps rather less 
accurate determinations. 
In repeated parallel experiments with persons of different types 
of colour vision the same result has been always verified again. There 
is no known form of anomalous colour vision for which any noticeable 
deviation has been found in the distribution of twilight values over the 
spectrum. The only cases that have not yet been tested are those of 
pure tritanopia or violet blindness. 
A. Tschermak1 has stated over and over again that the adaptative ability 
of the eye for long and short wave-lengths for the two principal types of colour 
1 A. Tschermak, Über physiologische und pathologische Anpassung des Auges. Leipzig 
1900.—Beobachtungen über die relative Farbenblindheit im indirekten Sehen. Pflügers 
Arch. f. d.-ges. Physiol. LXXXII. 559. 1900.—Die Helldunkeladaptation des Auges und 
die Funktion der Stäbchen und Zapfen. Ergebnisse d. Physiol. I, 2. 703 and 747. 1902.


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