THE VISUAL PIGMENTS 



affected. Thus any change in visual purple density would upset the 

 null balance of the galvanometer. This could then be restored by a 

 suitable movement of a purple wedge, W, in Fig. 7.10, placed in the 

 path of the incident light. In this way any decrease or increase in 

 visual purple could be followed by noting the purple-wedge positions 

 required for balance. 



Measurements of the changes which occurred when the dark- 

 adapted human eye was bleached were made at a number of retinal 

 positions. At the fovea and the blind spot there was no change on 

 bleaching, indicating the absence of visual purple from these retinal 

 positions. In other places, however, there was a rapid exponential 

 fall in density, similar to that found in experiments with the rabbit 

 (Fig. 7.7 (A)). A measure of the visual purple density in different 

 regions was provided by the difference between the wedge readings 

 for the dark-adapted and light-adapted retina. These differences 

 were ascertained for a number of positions along the horizontal 

 meridian from 45° temporal to 40° nasal. The results are shown by 

 the black dots in Fig. 7.11 and compared with osterberg's (1935) 

 histological determination of rod density over the same region of the 

 human eye. 



London^ August 1955. 



REFERENCES 



Abelsdorff, G. (1896). Die ophthalmoskopische Erkennbarkeit des 



Sehpurpurs. Z. Psychol. Physiol. Sinnesorg., 14, 77-90. 

 Abelsdorff, G. (1898). Physiologische Beobachtungen am Auge der 



Krokodile. Arch. Anat. u. Physiol. (Physiol.), 155-167. 

 Arden, G. B. (1954a). Light-sensitive pigment in the visual cells of the 



frog. /. Physiol., 123, 377-385. 

 Arden, G. B. (1954b). The dark reactions in visual cell suspensions. 



/. Physiol., 123, 386-395. 

 Arden, G. B. (1954c). A narrow-band pigment present in visual cell 



suspensions. /. Physiol., 123, 396-408. 

 Boll, F. (1876). SuU' anatomia e fisiologia della retina. Mem. Acad. 



Lincei, 1, 371-393. 

 Brindley, G. S. and Willmer, E. N. (1952). The reflexion of light from 



the macular and peripheral fundus oculi in man. /. Physiol,^ 116, 



350-356. 

 Broda, E. E., Goodeve, C. F. and Lythgoe, R. J. (1938). The weight of 



the chromophore carrier in the visual purple molecule. J. Physiol., 98, 



397^04. 

 Campbell, F. W. and Rushton, W. A. H. (1954). The measurement of 



rhodopsin in the human eye. /. Physiol, 126, 36-37P. 



204 



