W. A. H. HUSHTON 



717 



With a 10% range of pigment to investigate and an accuracy of 

 measurement which at best is 3%, the prospect of estabhshing any 

 {piantitative relation between pigment and threshold seemed small. 

 But we were fortunate in obtaining the cooperation of an excep- 

 tionally suitable (incomplete) rod monochromat. She appeared to 

 iiave normal rhodopsin, no measurable foveal pigments, and no func- 

 tional visual receptors except rods off the fovea, though she certainly 

 had a few cones upon the fovea with which she could fixate. 



Fig. 5 shows results of experiments by Fuortes, Gunkel, and me 

 upon this subject. The black circles show the rhodopsin regeneration 

 curve as compared with normal (Fuortes, white circles) . The irregu- 

 lar tracing is the dark adaptation curve traced by the subject on 

 Gunkel's apparatus. In the absence of cones there is no early branch 

 to hide the enormous rise in rod threshold, which in this tracing 

 starts at 3,000,000 times the dark-adapted value. The lower part of 

 this curve coincides with the rod portion of Fuortes's curve, obtained 

 following a similar (total) bleach, and indicates that rod function 

 appears at a given level at the same moment whether there are cones 

 or not; hence cone inhibition plays no part in depressing rod function 

 here, whatever may occur in other aspects of visual function. 



The relation between threshold and pigment for the monochromat 

 is sufficiently extended to be worth something. It is plotted in Fig. G 

 and demonstrates the linear relation between pigment bleached and 

 log threshold that Wald has long insisted upon, and which he has 

 deduced as a consequence of his "compartment hypothesis" (13) . 



^ Rod Monochromat 

 -Q. Normal 



green. .^ 

 yellow/ '■'.;. 



^-A 



10 



20 

 Minutes 



30 



10 

 9 

 ■8 ? 



6 2, 



3 5 



40 



Fig. 5. Regeneration of rhodopsin in normal ami rod monochromat (while and 

 black circles, right scale). Dark adaptation curve traced by monochromat over 

 6.5 log units (continuous irregular line, left scale). The dotted line shows dark 

 adaptation in the normal, changing from cones to rods (green test) at 18 min and 

 continuing along the monochroniat's curve. 



