Volume 5

Treasury of human inheritance / edited by Karl Pearson, R.A. Fisher, L.S. Penrose.

Date:
1909-
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Treasury of human inheritance / edited by Karl Pearson, R.A. Fisher, L.S. Penrose. Source: Wellcome Collection.

    39/190 (page 155)
    only of the chief properties demonstrated to hold for this substance. The most sig nificant feature of visual purple is its sensitiveness to light; it bleaches under the action of light and the intensity of the action varies for different wave lengths, and is for most creatures at a maximum for a wave length of 500 /rg; in fishes the action is at a maximum at a wave length of 540 p,/r, which is of special interest if we believe, following Hess, that fish are colour-blind. The following very striking diagram suffices to indicate that there are close relations between the thresholds of vision of the dark- adapted human eye, the absorptions of light by the human visual purple and the energies absorbed in bleaching rabbit’s visual purple for different spectral lights. If we invert the Diagram 8. After V. Henri and Larguier des Bancels, Journal de Physiologie el de Pathologie générale , T. 13, pp. 841-56, 1911. Curve a gives the relative luminous energy needful at the threshold of vision for the dark-adapted eye, from the observations of Schaternikoff on man. Unit energy at 509 g/i. Curve h. Quantities of light which must fall upon the same solution of human visual purple for this solution to absorb the same quantity of energy, from the observations of A. Konig. Unit of light at 500 g.fj,. Curve c. Quantities of energy producing the same amount of bleaching in the visual purple of rabbits, from the observations of Trendelenburg. Unit energy at 509 yy. (Trendelenburg determines the “velocities of bleaching” by the reduced absorption of the visual purple after timed exposures to the given light. One would like to have a curve for the first sensible bleaching for comparison.) curve for bleaching, we obtain the “bleaching values” of the several spectral lights and this will clearly follow closely the human achromatic scotopic luminosity curve, and accordingly the luminosity curve in a good light of the totally colour-blind man. We cannot say what significance, if any, can be attached to this association, or whether on the basis of it the possibility of visual purple being primarily concerned with vision in a dim light is a reasonable proposition ; the extreme rarity of the substance in certain nocturnal animals is against such a view, but again, on the other hand, since the substance reacts strongly to blue light and hardly at all to red light, the proposition might explain how in the evening blues become relatively more conspicuous and reds