N. 297, 298.] B. Duplicity Theory and Twilight Vision 353 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.