Sensory Discrimination: Vision 127 



If the worm is detached from the burrow, it will take a course 

 leading it more or less obliquely away from the light ; if it is 

 crawling in passages between glass plates, which allow it the 

 choice between only two paths, one straight toward the light 

 and the other straight away, it takes the latter about 95 per 

 cent of the time (38 7 ). Graber used his ' ' Preference Method ' ' 

 on earthworms, employing a box with two compartments, one 

 illuminated with diffused daylight, the other dark. At the 

 end of every hour the number of worms in each compartment 

 was counted. That in the darkness was on the average 5.2 

 times as great as that in the light. When ground glass was 

 substituted for the dark screen, making the compartment 

 under it about half as light as the other, the number in the 

 lighter compartment was about .6 the number in the darker, 

 though still moderately light, portion of the box, thus show- 

 ing that the worms were sensitive to comparatively small 

 differences in intensity. Graber also placed colored glasses 

 over the compartments, with the following results : the worms 

 preferred red to blue even when the former was much lighter 

 than the latter, indicating that the preference was determined 

 by the wave length and not by the brightness of the light; 

 they preferred green to blue under similar conditions, and red 

 to green. They emphatically preferred white light from 

 which the ultra-violet rays had been subtracted to ordinary 

 white light, 6.7 times as many being found in a compartment 

 covered by a screen impervious only to ultra-violet rays (150). 

 The effect of ultra-violet rays on many animals is very dele- 

 terious (167). The avoidance of the ultra-violet rays and 

 the seeking of red by negatively phototropic animals is almost 

 universal. 



The part of the body of the earthworm affected by the light 

 influences the reaction. Darwin indeed reported that the 

 worms withdrew into their burrows only when light fell on 



