[neelin] the sensitiveness OF THE EYE TO LIGHT 233 



angle which is necessarily less than 90°. Now Cos-90 = and Cos^Oj 

 which is slightly greater than will, as explained in the fore part of 

 this paper, represent the amount of stimulus necessary to produce a 

 sensation of light. This amount of light (Cos'aj) , as an initial stimulus 

 was allowed to fall upon the retina and the polariser further rotated. 

 As soon as the sensation of light as experienced from the initial stimu- 

 lus was perceived to change rotation was again stopped, and the angle 

 indicated on the scale read. If a^ denote this angle, Cos-a.^ will repre- 

 sent the amount of light going through the niçois when rotation was 

 stopped. But Cos-Oj, represents the amount of light going through 

 the niçois before the last rotation began. Therefore (Cos"a2— Cos-aJ 

 will represent the amount of light by which the initial stimulus 

 (Cos'ttj) was increased to produce a change in the sensation that was 

 just perceptible. Observing again the amount of light now in the 

 eye-piece (Cos'aj) and further rotating the polariser until as before 

 another change of sensation was experienced we obtained the angle 

 O3. Since in this case Cos^ a2 represents the initial stimulus (Cos'ttg 



— Cos-aj) will represent the increment of light necessary to produce 

 a change of sensation that was just perceptible. In this way the light 

 going through the niçois at the end of each observation was the 

 light upon which each subsequent observation was made until full 

 brightness was reached. That is: the intensity of light used for any 

 observation is represented by Cos-a^ and the increment added by 

 (Cos-ttj^ ^ J — Cos- Ojj) where n represents the number of obser- 

 vations made from the crossing of the prisms. In this way several 

 portions of the spectrum were examined. The radiation observed in 

 each portion varied only in intensity and hence the sensibility to radi- 

 ation of varying intensity was obtained. The reciprocal of (Cos-a^ ^ j 



— Cos-a^), taken to represent sensibility, was plotted as a func- 

 tion of the corresponding initial intensity, Cos^ a^. The re- 

 sults are shown in Fig. 5. The radiation used in obtaining the 

 different curves is indicated in the figure. The portions of the 

 spectrum observed with the data obtained and the calculations 

 made from them are given in tables VII, VIII, X, XII, XIII 

 and XIV. 



Starting with the spectrum at maximum brightness and observ- 

 ing the least perceptible decrease in intensity at each step till com- 

 plete darkness was reached, data were obtained from which the curves 

 indicated in Fig. 5 as "curves of decrements," have been plotted. 

 These data are to be found in Tables IX and XI. These curves 

 seem to indicate that the increase in sensibility with decreasing 

 intensity follows the law for decreasing sensibility with increasing 



intensity. 



Sec. Ill, 1913—15 



