[neelin] the sensitiveness OF THE EYE TO LIGHT 235 



that was just perceptible. In the other tables of this part "Least 

 perceptible increment" is the amount Idj which the intensity of the 

 light was increased in effecting a change of sensation that was just 

 perceptible. "Sensibility" is the value of the reciprocal of the least 

 perceptible increment, or decrement, as the case may be. 



III. 



Sensibility to small differences in Intensity, 



1. Without further observation it may be shown from the data 

 in Tables VII to XIV that the least perceptible increment or decre- 

 ment measured as a fraction of the whole is approximatel}^ independent 

 of intensity. 



It has already been shown (II) that, with any constant wave 

 length, the intensity of the light used for any observation say, the 

 nth, may be represented by Cos'a^ and the least perceptible incre- 

 ment by (Cos-a^j ^ ^ —Cos- a J . Let 



Cos-a ,, — Cos-a ^ 



— —- =k or Cos-a^ + j - Cos-a^ =k Cos-a^ + ^ 



e-os'a^ + 1 



There appears to be a difference of opinion as to whether the de- 

 nominator here should be Cos-a^ or Cos-a^^ ^ j . Substituting the 

 proper values for a^, curves have been plotted as shown in Fig. 6. 

 It was impossible in some cases to mark all points on the curve be- 

 cause of their coming so close when the intensity was low and so repre- 

 sentative points are shown. 



2. Again, from the data given in Table XV it may be shown 

 whether "the least perceptible increment measured as a fraction of 

 the whole is approximately independent of the wave length (Konig's 

 Law) at constant luminosity." For since the data given in this table 

 were obtained by making observations upon different portions of what 

 was thought to be the equivalent of a spectrum of uniform luminosity, 

 we may infer that while the wave length varied with each observation, 

 the intensity of the light as observed in the eye—piece of the spectro- 

 meter remained constant for all observations. Take for example, the 

 data furnished by the third observation in the table referred to above. 

 It is shown there that the amount of light going through the niçois 

 at the beginning of the observation is represented by -1170, while 

 the amount of light going through at the end of the observation is 

 represented by -1849 showing that -0679 of the total intensity of the 

 spectrum where the wave length equaled '491/^, had to be added to 



