4 
352 PHYSIOLOGY: W. F. HAMILTON Proc. N. A. S. 
the "fatiguing" light arranged to "stimulate" and vice versa. In the 
second test, the animals were, of course, not dark adapted. 
If, however, the two Hghts were of the same or nearly the same wave- 
lengths the results were quite different. The lights were balanced, the 
"stimulating" light intercepted, and the animals subjected to the "fa- 
tiguing" light. There was evidence of fatigue, but when the "stimulating" 
light was again allowed to fall on the flies, it was easily seen that the fatigue 
was not differential. The animals distributed themselves at random, 
between the two ends, just as they did when the lights were first balanced. 
The spectrum was then scaled. The smallest difference in wave-length 
which showed conclusive evidence of differential fatigue was the unit used. 
Beginning at 385 Wju, which is as low as the calibration of the instruments 
extend, the values were found as follows : 
385-410, 410-430, 430-450, 450-500 nifx 
The last determination was rather doubtful. At the intensity available 
the orienting effect of light above 500 ntfi was insufficient to give conclu- 
sive evidence of differential fatigue. The distribution of these points and 
the fact that "hue perception" is at its maximum between 410 w/x and 
450 m }i makes it appear that there may be two receptor systems, one for 
the blue- violet and one for the blue-green. Evidence for a third compo- 
nent seems to be lacking. According to Steindler^ the hue perception 
curve of the normal human eye indicates the existence of three receptor 
systems. 
3. It is planned to extend this work to other organisms and to work 
out the energy — time relationships of differential fatigue with different 
pairs of wave-lengths at different intensities. 
The method should prove of value in analyzing the wave-length recep- 
tors of lower organisms, and its modifications and applications are numerous. 
It is applicable to the investigation of the pupil reactions, of the action 
current of the optic nerve and to similar problems. Perhaps one of its 
most interesting applications would be to ascertain the course of evolu- 
tion of the color sense among lower organisms by means of photic orien- 
tation in crossed beams of light. The chief value of the method is its 
objectiveness. 
1 Laurens, H., and Hooker, H. O., 1920, Studies on the relative physiological value of 
spectral lights. J. Exper. Zool, 30, 345-68. 
2 Koenig, Ges. Abhandlungen, Berlin. 
3 Edrige Green, F. W., 1920, The Physiology of Vision. London. 
^ Houstoun, R. A., 1916, A theory of color vision. Proc. Roy. Soc. London, A92, 
424-32. 
5 Burch, G. T., 1899, On artificial temporary color blindness with an examination of 
the color sensation of 109 persons. Proc. Roy. Soc, B191, 1-34. 
6 Abney, Sir W. de W., 1913, Researches in Color Vision. London. 
