206 COLOUR VISION 



visual purple are desirable. Judging by the position of the maximum 

 it should approximate more nearly to the type found in fishes. It 

 should be borne in mind that visual purple has not been isolated in a 

 pure state, free from hemoglobin. 



Trendelenburg's curve for bleaching values of the frog's visual 

 purple agrees more closely with the achromatic scotopic luminosity 

 curve of the human eye (Fig. 1). 



There can be no doubt that the visual purple is of fundamental 

 importance in scotopic vision and that its accumulation is the basis 

 of dark adaptation. It follows that the relation of the achromatic 

 scotopic luminosity values of two lights, e.g. a green which is strongly 

 absorbed and an orange which is weakly absorbed by the visual purple, 

 should depend upon the concentration of visual purple in the rods, 

 i.e., it should vary with the degree of dark adaptation. Stegmann 1 

 has shown that such is the case. If a luminosity match is made between 

 green and orange with intensities at which they appear colourless after 

 5-] 5 minutes' dark adaptation, and the lights are again compared after 

 much more prolonged adaptation the orange appears much brighter 

 and must be diminished to about three-quarters of its former intensity 

 to restore the match. It is to be noted that the change is in the 

 opposite direction to that of the Purkinje phenomenon. 



Sachs 2 showed that the pupil-reactions vary with the luminosities 

 of the lights, independently of their colour, both in the normal and 

 colour-blind. Abelsdorff 3 confirmed these observations and showed 

 further that maximum pupil-constriction occurs in nocturnal animals 

 from green light, whereas in animals of diurnal habits the maximum is 

 obtained from yellow light. 



Similar, but more reliable and much more striking results, have been 

 obtained from the electrical reactions, especially in day- and night-birds 

 by Himstedt and Nagel 4 and Piper 5 . 



Vision of lights of low intensities with the dark-adapted eye is char- 

 acterised by (1) the absence of colour sensations, (2) a greatly increased 

 sensitiveness for lights of low intensity, (3) a relatively increased 



1 Dissertation, Freiburg, 1901. 



2 Arch. f. d. ges. Physiol. LII. 79, 1892 ; Arch. /. Ophth. xxxix. 3, 108, 1893 ; Ztsch. f. 

 Psychol. u. Physiol. d. Sinnesorg. xxu. 388, 1900. 



3 Ztsch. /. Psychol. u. Physiol. d. Sinnesorg. xxu. 81, 451, 1900; Arch. f. Augenhlk. 

 XLI. 155, 1900 ; Arch. f. Anat. u. Physiol. 541, 1900. 



4 Ber. d. naturf. Ges. Freiburg, n. 1901 ; Ann. d. Physik, iv. 1901 ; in v. Helmholtz, 

 3rd ed. n. p. 328. 



5 Arch. f. Anat. u. Physiol. 543, 1904. 



