382 WAVE-LENGTH AND PHOTOSENSORY PROCESS 



mediately in contact with the shutter (for a description of the appara- 

 tus used, see Hecht, 1919-20, c). Several animals which have been 

 thoroughly dark-adapted are then used. The reaction time of each 

 animal is measured with a stop-watch. After a rest of 15 minutes in 

 the dark, the animals are again tested, but at a distance from the 

 light nearer or farther than before, depending on the magnitude of 

 the previous reaction time. After another period in the dark, the 

 reaction time is measured at still another intensity, and the process 

 repeated until enough responses have been measured to cover a range 

 between 2.0 and 4.0 seconds in the reaction time. 



The curve giving the relation between the reaction time and the 

 intensity is the //-isochrome for this filter. The entire procedure is 

 then repeated with another filter, until all the filters have been tested. 

 In this way a series of //-isochromes are mapped out for the seven 

 portions of the visible spectrum shown in Fig. 3. 



III. 



1. Several preliminary experiments agreed in showing that the light 

 coming through Filter 75 is the most effective portion of the spectrum. 

 A final set of experiments was then arranged using nine dark-adapted 

 animals. The results are given in Fig. 4. Each point is the average 

 of nine determinations of the reaction time, one for each animal. The 

 curves are the //-isochromes drawn smoothly through the points. An 

 exception is the curve for Filter 76, which for obvious reasons is drawn 

 parallel to the other six curves. 



The range of intensities required to produce similar responses is very 

 large; from 2 to 16,000 units. To present them in a single figure is 

 manifestly impractical. Moreover, it has been shown that the reac- 

 tion time is inversely proportional to the logarithm of the intensity. 

 Consequently, the abscissas of Fig. 4 are the logarithms of the inten- 

 sities and not the intensities themselves. The use of such a logarith- 

 mic plot makes all the //-curves parallel to one another, and renders 

 their comparison more simple than would otherwise be the case. 



The //-isochromes of Fig. 4 show at once that the most effective 

 portion of the spectrum is the light transmitted by Filter 75. This 

 corresponds to a band whose maximum is at 490 ii/j., in the blue-green. 



