STUDIES IN SPECIAL SENSE PHYSIOLOGY 375 



partment of physiology is there a greater tendency to advance 

 equivocal evidence in favour of an hypothesis than in that con- 

 cerned with the special senses. 



I can now sum up the case presented. I hope I have rendered 

 it probable that 



(1) There is a marked difference between central and peripheral 

 vision, in regard to the phenomenon of darkness adaptation, the 

 former being little if at all affected in the process. 



(2) These differences may be provisionally interpreted on the 

 supposition that visual sensations are bound up with two distinct 

 mechanisms : (a) That of the cones, with which chromatic sensitivity 

 and achromatic sensations under daylight conditions are associated ; 

 (6) that of the rods upon which depend achromatic sensations 

 under conditions of dark adaptation. 



The objections to this view are neither few nor unimportant. 

 It has not been proved that no central adaptation whatever occurs. 

 The equations (colour matches) of totally colour-blind persons and 

 those of the normal " dark " eye agree well but not absolutely. 

 There is also a difficulty in interpreting the secondary image of 

 recurrent vision. 



That the first of these objections (as well as the kindred one 

 that a central scotoma does not exist in all cases of total-colour- 

 blindness) may be parried by supposing a trace of visual purple 

 and a few scattered rods to be present in the fovea, is clear. Recent 

 measurement by Fritsch ( 17 , p. 188) on a negro's fovea gave an 

 absolutely rod-free zone of only '2 mm., corresponding to an 

 angular distance of less than a degree ; we could not allow much 

 weight to failures in the demonstration of such small adaptable 

 areas, even supposing them to be absolutely rod-free. 



The difficulty regarding " total-colour-blinds " and normal 

 " dark " equations is not formidable. The fact is that sufficient 

 measurements have not been made to enable us to affirm that the 

 differences are significant. The more serious question as to the real 

 significance of the secondary image in recurrent vision has been 

 already considered ; it may prove the crucial point in the theory. 

 Accepting the above view of the role of the visual purple and rods 

 as an important element in the physiological processes of vision 

 with low intensities of light, one is tempted to speculate as to the 

 nature of their activity. Parinaud was of opinion that the process 

 depended upon fluorescence of the retina mainly due to the presence 



