THE ANATOMICAL BASIS 13 



state that it can be seen entoptically on waking in the morning as a 

 rose-red ring round the fixation point, projected against the white 

 ceiling. Haab attributes this phenomenon, not to the visual purple, 

 but to the pigment of the yellow spot. Edridge-Green^ confirms Tait's 

 and Boll's observations, and states that the purple can be seen to flow 

 in waves from the periphery of the yellow spot towards the point of 

 fixation. 



The colour of the visual purple varies in different animals and under 

 different circumstances^, and gives different spectroscopic absorption 

 bands (Kiihne ; Kottgen and Abelsdorff^). The maximum absorption 

 in fishes is in the yellow (540 ft/u,), in mammals in the blue-green (500 M/i) ; 

 hence it is reddish violet in fishes and purple in mammals. The occur- 

 rence of " visual yellow " as an intermediate stage in the bleaching 

 of visual purple (Kiihne) seems to have been conclusively disproved by 

 Kottgen and Abelsdorff. They found that as the visual purple was 

 bleached the relative absorption remained unchanged. The substance 

 therefore becomes gradually less concentrated, without passing through 

 a yellow stage. 



The bleaching of visual purple is limited to the area exposed to 

 light, so that an optogram or image of the luminous object, such as a 

 window, can be obtained. Such an optogram can be partially preserved 

 by alum solution, somewhat as a photographic negative is fixed, though 

 the processes are entirely dissimilar. Two to seven minutes' exposure 

 to light suffice to obtain a good optogram in the frog's retina. Light 

 on one eye does not cause any bleaching of the visual purple of the 

 other. 



The bleaching of visual purple by monochromatic light has proved 

 to be of great theoretical interest. Observations have been made by 

 Konig^ Kottgen and Abelsdorff, and Trendelenburg^. Trendelenburg 

 took two specimens of frog's visual purple and exposed one to Hght of 

 the sodium line (D, 589 /x/x) and the other to another Hght from the 

 same dispersion spectrum, the diminution of absorption being measured 

 by the spectrophotometer. The human achromatic scotopic luminosity 



1 J. of Physiol. XLi. 263, 1910 ; xlii 428, 1!)11 : xlv. 70, 1913. 



'^ Garten, in Graefe-Saemisch Handb. d. ges. Augenheilkuride, Teil i. Bd. ni. Kap. 

 xii, Plate VII, 1908. 



' Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xn. 161, 1896. 



* Konig, p. 388. 



'" Centralhl. f. Physiol, xvii. 1904 ; Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxxvn. 

 1, 1904. 



