Mast, Light Reactions in Lower Organisms. 141 



The field of light in which the colonies were exposed in the above 

 experiments was high in intensity at either end and low in the 

 middle. In such a field of light it is clear that an organism 

 swimming toward the light in the middle is stimulated alike on 

 both sides, since the lateral rays necessarily come in equal numbers 

 from both ends of the field. Consequently the direction of motion 

 cannot be influenced by these rays. But if the organism in travel- 

 ins toward the light swims nearer one end of the field than the 

 others, the lateral rays might influence the direction of motion. 

 If, however, the lateral rays do affect the direction of motion under 

 such conditions, we should certainly expect to be able to detect it 

 when all lateral rays on one side of a colony swimming toward 

 the source of light are eliminated by shading the entire portion of 

 the field either to the right or to the left of the colony. I repeated 

 the above experiments many times with a portion of the field thus 

 shaded, but was unable to detect any effect on the angle of deflec- 

 tion. It must therefore be concluded that the difference in deflec- 

 tion, represented in columns i and 11 of Tables II and III, was not 

 caused by lateral rays. 



The direction of motion in Volvox exposed to light is 

 consequently regulated by the relative intensity of the 

 light on opposite sides of the colonies regardless of the 

 direction of the rays. 



Cause of the Effect of Change m Intensity Upon the Degree of 

 Deflection in Graded Light. — The difference in intensity of illumina- 

 tion on opposite sides of the colonies exposed in the light grader 

 under the conditions of the experiments just discussed, can readily 

 be calculated. The light intensity was 442.6 candle meters at 

 either end of the field, from which it gradually decreased toward 

 the middle, where it was 57 candle meters. The distance from 

 the middle to either end was 60 millimeters. We have therefore a 

 change of 385 + candle meters in 60 millimeters or 6.4 candle 

 meters per millimeter. The largest colonies are nearly a milli- 

 meter in diameter and the average light intensity to which they 

 were exposed was about 333 candle meters. In the largest speci- 

 mens, then, one side was exposed to an intensity of about 330 and 

 the other to an intensity of about 336 candle meters. 



If Weber's law holds true, as we have good reason to believe, 

 (see p. 171), we should expect this difference in intensity on oppo- 

 site sides to be more effective in weak light than in strong and we 

 should consequently expect a greater deflection in regions in the 



