Literary Xoticcs. 373 



1:9000. The explaiiiition for this eiiornious increase in sensitivity is 

 to he found in tin- la( k of rods in the central region of the retina. The 

 rods are tised in weak light, while the (ones are used in strong lights. 

 Hut it eunnot he supposed that the stimulus threshold for the 

 eones remains the same under all c ireumstances. Is not the sensitivity 

 decreased by long activity and increased by rest? The difficidties iu 

 the wav of a satisfactory answer to these (luestions are almost insur- 

 mountable. According to the authors, however, there are are three 

 possible methods of approach : 



1. One (an arrange a fixation mark of minimal size and bright 

 enough to be securely above the foveal threshold. A steadfast fixa 

 tion point is thus secured. This is very necessary since at the moment 

 of the onset of adaptation for darkness the greater sensitivity of the 

 peripheral portion of the retina causes one to fixate with the periphery 

 rather than with the fovea. 



2. One can make use of the fact that the adaptive increase in the 

 sensitivity of the rod apparatus is least for |Mire red light ; and is the 

 smaller, the longer the wave length of the light. In consequence of 

 this a foveal threshold can be determined without danger of the point 

 of regard being turned towards the periphery. 



3. Since it takes 5 minutes for the process of adai)tati(Mi for dark- 

 ness (after the eye has been adapted for light) to set in, it is possible, 

 in the first few moments after entrance into the dark room, to obtain a 

 foveal threshold without the fixation mark being diverted either to the 

 l)aracentral or to the peri|)heral portion of the retina. 



.Summarv of results (detained from all three methods. 

 .\ . Investigation-; with red light. 



1. With foveal and para( entral region. 



a. When the adaptation for brightness is produced bv strong arti- 

 fi( ial light, tlic iiK reasc in scnsitivitv during the first minute is about 

 thirtv-two fold. I'rom the end of the first _^o se( onds to the end of 

 the sixth minute ue have the ratio of 1:1'). 



1). Wiicn the adaptation lor brightness is producecl bv bright sun- 

 light, the in< rease in seiisitiv ity, from first moment of entrance into 

 the dark room, to the onset of adaptation for darkness, is in the ratio 

 of \:2oo. 



2. \\ ith the foveal region alone a fourfold increase in sensitivity 

 is found. 



H. ln\ estigations with green and iilue lights. 



I. With tlie toveal region alone the increase in sensitivity is again 

 fourfold. 



