[neelin] the sensitiveness OF THE EYE TO LIGHT 225 



was rotated, the difference between 1 and Cos- a i.e., Sin^ a will repre- 

 sent the amount by which the original intensity of the light was 

 diminished in effecting a change of sensation that was just perceptible. 

 The reciprocal of this value (sin- a) taken to represent the sensibility 

 of the eye has l^een plotted as a function of the corresponding wave 

 length. This is shown in Curve 1, Fig. 2. The other chosen portions 

 of the spectrum were observed in rapid succession in a similar way. 



A dimmer spectrum was next considered. This was obtained by 

 setting the polariser before each observation so that its principal plane 

 made an angle of 51° with the horizontal, i.e. with the plane of the 

 analyser. This initial angle was denoted by /3. Since Cos ^/9 = Cos^ 

 51° is equal to -395, it was considered that each portion of the spectrum 

 chosen- for investigation was 39-5% of the intensity of corresponding 

 portions in the spectrum at maximum intensity. With this initial 

 arrangement light as observed in the eye-piece was, as before, allowed 

 to act upon the retina for four seconds. Then the polariser was rotated 

 so as to increase the intensity of the light. As soon as the intensity had 

 just noticeably increased the rotation was stopped. The angle between 

 the planes of the niçois was read and denoted by a. Since Cos-51° 

 represents the mtensity of the stimulus at the begining and Cos- a the 

 intensity of the light affecting the eye at the moment of perceptible 

 change, Cos-a — Cos^51° will represent the amount by which the original 

 intensit}^ (which for each observation is 39-5% of the maximum 

 intensity of the spectrum at the chosen-point) was increased in affecting 

 a change of sensation that was just perceptible. As before, the recipro- 

 cal of this value was taken to represent the sensibility of the eye and has 

 l^een plotted as a function of the corresponding wave length. The 

 results are shown graphically in Curve II, Fig. 2. The portions of the 

 spectrum observed and the observations made upon each are given 

 in Table II. 



Upon the spectrum of intensity -25 observations were made to 

 determine the least perceptible decrease in the initial stimulus necessary 

 to produce a noticeable change in the sensation of light. This was 

 done by following the method descril^ed for the spectrum at maximum 

 intensity. The results are given in Table III and are shown graphically 

 in Curve III, Fig. 2. The spectrum of intensity «25 was obtained by 

 setting the plane of the polariser at an angle of 60° with the plane of 

 the analyser. 



In the case of other spectra investigated observations were made 

 for the least perceptible increase in the initial stimulus, following the 

 method used upon the spectrum second in order of brightness, as de- 

 scribed above. These spectra were of relative intensities -OGO; '025 

 and «0054 respectively, — the spectrum of maximum intensity beino- 



