352 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



eye to a stimulus largely depends upon whether, before the ex- 

 periment, the subjects have rested in a dark room or been exposed 

 to light that is to say, whether there be a condition of light or 

 dark adaptation. 



Many years ago, Aubert noticed that the response of the eye 

 to feeble stimulation was increased by resting in the dark, and also 

 lajd down the rule that the threshold value of a stimulus in such 

 cases varied inversely as the area of surface stimulated. This 

 general statement received ample confirmation ; Charpentier ( 1 ), 

 for instance, found that the central part of the " dark " l retina, 

 although exhibiting an increased response as compared with the 

 same region in the " light " eye, was far less sensitive than the 

 periphery. This increased responsiveness was very marked in 

 respect of the short waved regions of the spectrum, indeed it 

 is denied by many good observers that adaptation affects the 

 response to red light at all (Parinaud and v. Kries), while nobody 

 has demonstrated a large increase. 



Of the many who have studied the changes of responsiveness 

 in adaptation, all agree that they are far more prominent in the 

 peripheral than in the central region, but much dispute has arisen 

 as to whether any part is absolutely unaffected. On the whole, 

 it may be said that no satisfactory proof is forthcoming that 

 such a part exists. Experimentally, the problem is beset with 

 difficulties, for it is hard to be certain of the exact limits of a very 

 small stimulated field, and it is possible that the time required for 

 adaptation is not the same in the fovea and the peripheral regions, 

 being probably longer in the former (Tschermak 2 ).. 



These changes in responsiveness immediately lead us to the 

 consideration of Purkinje's "phenomenon," an effect which may 

 be described in the following terms. If one examines an ordinary 

 spectrum, the brightest part of it seems to occupy the neighbour- 

 hood of the yellow or orange-yellow ; if now its physical intensity 

 be diminished for instance, by moving the source of light farther 

 away from the prism or grating the maximum of brightness is- 

 shifted towards the violet end. With the feeblest illumination 

 which enables us to detect the spectral colours at all, the brightest 

 part is at the junction of the green and blue. 



Hering ( 3 ) by means of a simple experiment has made it seem 



1 For brevity, I shall speak of an eye which has been rested in darkness as a. 

 "dark " and one previously exposed to light as a " light" eye. 



