Sensory Discrimination: Vision 145 



violet end. He also sees in place of the red a gray darker 

 than the brightness of red to the normal eye would lead 

 one to expect. This altered distribution of brightness 

 in the spectrum occurs for the normal eye also, under 

 very faint illumination : in twilight the spectrum looks 

 to the normal eye just as it does to the totally color-blind 

 eye, a band of grays brightest in the yellow-green region. 

 If we had no other means of deciding whether or not a 

 man was color-blind, we should take as evidence of color- 

 blindness the fact that for him the brightest region of the 

 spectrum lay in the yellow-green rather than the yellow, in 

 ordinarily bright light. It is therefore of some impor- 

 tance to the problem of color vision in the lower animals 

 to find how strongly the light rays of various wave-lengths 

 affect them. But we must bear in mind that for the lower 

 animals it is impossible to conclude color-blindness from 

 the fact that the brightness values, that is, the effective 

 intensities, of the different colors are what they would be 

 for a color-blind human being. Just this unsafe inference 

 is, however, drawn by certain authorities. 



In plants, the maximum effect of colored light is exerted 

 by the rays at the violet end : violet, indigo, or blue. The 

 problem has been investigated for microscopic animals by 

 an arrangement such that two beams of light fall on the 

 organism at right angles to each other. Now if the or- 

 ganism has a tendency either to seek or to avoid light, 

 and if the two beams are of equal intensity, the animal will 

 move on a diagonal between the two beams. If either 

 ray has a stronger effect than the other, the course of an 

 animal which seeks light will be inclined towards the more 

 effective beam; that of an animal which avoids light, 

 towards the less effective beam. If the two beams are of 

 different colors, it will thus be possible to test the stimu- 



