384 



WAVE-LENGTH AND PHOTOSENSORY PROCESS 



energy required to produce the same sensory effect. The value of the 

 latter method will become apparent when the attempt is made in the 

 next section to interpret the experimental findings. 



In the present instance the same sensory effect is represented by 

 responses produced in the same reaction time. It is therefore required 

 to measure the different intensities necessary to produce the same 

 reaction time. This can be done by reading from the //-isochromes 

 of Fig. 4 the intensities corresponding to definite values of the reaction 

 time. 



I have done this for three values; for 2.5, 3.0, and 3.5 seconds. In 

 each instance the intensity for Filter 75 is placed at unity, and the 

 relative intensities of the others computed in terms of it. The three 

 values so obtained for each portion of the spectrum are given in 

 Table II. In treating them further, I have used the average of the 

 three figures for each filter. 



TABLE II. 



Relative Intensities Required to Produce the Same Reaction Time. 



3. The relative effectiveness of the different lights may be repre- 

 sented by the reciprocals of the relative intensities. Fig. 5 shows 

 these values in graphic form. For convenience, and for another 

 reason which will presently become apparent, the reciprocal of the 

 maximum is made equal to 100, the other values being computed 

 accordingly and employed as ordinates. The wave-length is used as 

 abscissa. Each point has been plotted as a rectangle, whose center 

 is the point of maximum energy transmission as given in Fig. 3, and 

 whose horizontal dimension represents the points between which 75 

 per cent of the transmitted energy is confined. A smoothed curve 

 through the points shows strikingly that the most effective portion 



