FUNCTIONS OF THE RETINAL STRUCTURES. 1103 



unpractised individuals perceived it easily. According to v. Kries, there 

 is no recurrent image in total colour-blindness (again only one case 

 examined) ; but this is denied by Hering and Hess. Haniaker's observa- 

 tions (p. 1074) seem to show that the phenomenon is complicated by 

 other processes going on at the same time. 



The slow rate of dark-adaptation upon which night-blindness depends 

 would be due to slow formation of visual purple. The two chief causes of 

 this condition, namely, disease of the retinal pigment layer, and action of 

 too strong light, would naturally affect the formation of the substance, 

 while its occurrence in jaundice and in diseases of the liver would be ex- 

 plained by the solvent action of bile salts on visual purple. Pilocarpine 

 is said to have a beneficial effect on night-blindness, 1 and Kiihne found 

 that this drug hastens regeneration of visual purple (p. 1045). Schirrner 2 

 has observed slowness of adaptation in cases of detached retina, and 

 compares this condition with the slowness of the auto-regeneration 

 described by Kiihne. 



Certain other facts are consistent with the theory. Exposure of one 

 eye to light has no effect on the dark-adaptation of the other eye, while 

 Engelmann found that exposure of one eye led to no change in the visual 

 purple of the other eye. G. E. Mtiller has found that the sensations 

 arising from electrical stimulation of the eyeballs are unaffected by the 

 condition of adaptation, and Kiihne found that visual purple was not 

 influenced by electrical stimuli. The fact that Dor (p. 1050) found a differ- 

 ence in the staining of rods and cones, after the action of red light, none 

 after blue light, is explicable if the former acts only on the cones, the 

 latter on both rods and cones. The observations of Koester and Konig 

 on visual acuity (pp. 1055 and 1086) are explicable, if the acuity of dark- 

 adaptation depends on the rods, that of bright adaptation on the cones. 



Foveal vision and absent visual purple. — The absence of rods 

 and visual purple from the fovea gives a means of verifying the 

 truth of the hypothesis. The difficulties of studying foveal vision have 

 been already mentioned. Another difficulty is the uncertainty as to the 

 anatomical limits of the area of the retina which contains no visual 

 purple. Most observers give the foveal depression a diameter of - 2 to 

 •4 mm., while others assign it larger values. For the present purpose, 

 however, the importance of the fovea lies in the exclusive presence of 

 cones. Comparatively few measurements have been made of the area 

 free from rods. Koster 3 has recently examined four cases, and found 

 the area in which rods were entirely absent varied from -44 to '901 mm., 

 while rods were comparatively scanty in a still larger area. Koster has 

 suggested - 5 mm. as the probable average diameter of the rod-free area, 

 and - 8 mm. as that of the area in which cones predominate. There is 

 still greater uncertainty as to the area from which visual purple is absent, 

 for Kiihne's observations seemed to show considerable individual differ- 

 ences, while, if his observation is correct, that the rods in the macula 

 are devoid of purple, still larger values than those of Koster must 

 be assigned to the area which has no visual purple. 



One feature of foveal vision about which there is no doubt, is the 

 high threshold for light. This low degree of sensitiveness to light, 

 which is especially marked in the dark-adapted eye, would be a natural 



1 Kuschbert, Deutsche med. Wchnschr., Leipzig, 1884, S. 324. 



2 Verhandl. d. X. intemat. med. Congress, Berlin, 1890, Bd. iv. Abth. 10, S. 63. 



3 Arch. f. Ophth., 1895, Bd. xli. Abth. 4, S. 1. 



