Sensory Discrimination: Vision 147 



subject of tests by the Preference Method, it was found 

 that if a choice was offered between two compartments, 

 one illuminated with diffuse daylight, the other dark, 

 and if the number of worms in each compartment was 

 counted at the end of every hour, those in the darkness 

 were on the average 5.2 as many as those in the light. If 

 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 of the 

 number in the darker, though still moderately light, com- 

 partment, showing that the worms were sensitive to com- 

 paratively small differences in intensity. When colored 

 glasses were placed over the compartments, the follow- 

 ing results were obtained : the worms preferred red to blue 

 even when the former was much lighter than the latter 

 to the human eye; 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 imper- 

 vious only to ultra-violet rays. It would thus appear that 

 in determining avoidance, blue hght is the most effective ; 

 on the other hand, Yung (833) finds the effect of colored 

 rays on the earthworm to be proportional to their intensity, 

 the green and yellow regions of the spectrum being most 

 effective. 



It is thus clear that when an animal discriminates between 

 rays of different colors, the discrimination may be based 

 merely on the intensity of the rays, either in themselves or 

 in the effect which they have on the organism, rather than 

 on their wave-length or color. Minkiewicz offers as evi- 

 dence of true color discrimination in a Nemertean worm, 

 Lineus ruber, the fact that he could alter its reactions to 



