358 HENRY LAURENS AND HENRY D. HOOKER, JR. 



the wave-lengths tested have a stimulating effect. With this 

 high-intensity spectrum it will be interesting to continue the 

 experiments into the longer and shorter waves. This it is 

 planned to do as soon as practicable. 



As mentioned earlier, the reaction time was also taken in 

 certain cases. That is the time that elapsed between the mo- 

 ment of exposure of the organism to the light, and the first indi- 

 cation of a locomotor response, or a swimming movement, to- 

 ward the source of the light. The results of computing the ratio 

 of the reaction time in the spectral light to the reaction time in the 

 white light show that the most effective wave-lengths (X494m/x) 

 are the same as ascertained from the determination of the* pre- 

 sentation time and of the rate of movement. The relative ef- 

 fectiveness of the various lights, due to the incompleteness of 

 the records, is, however, not clearly shown. 



We have used the time taken to traverse a certain fixed dis- 

 tance as an index of the relative stimulating efficiency of lights of 

 different wave-length. Although Volvox orients fairly precisely, 

 the colonies deflect somewhat, traveling in a wave-like course, 

 up and down or from side to side. This deflection is less when 

 the colonies are swimming toward the source of the lights of 

 greater stimulating effect. In other words, the degree of de- 

 flection varies with the stimulating efficiency of the light. We 

 did not note any consistent change in the precision of orientation 

 or of the rate of movement in the five tests given each colony. 



The relative time taken to swim a certain distance was not, 

 however, due merely to the amount of deflection or precision of 

 orientation, but also to the actual rate (strength) of swimming. 

 It was easy to see in the lights of low efficiency that the colonies 

 loafed along, now and then speeding up a little to slow down- 

 again, and so on. These variations in the energy output, or 

 the speed of swimming movements, being irrespective of the 

 distance which they had traversed or as to how near they were 

 to their goal. The conclusion seems justified, therefore, that 

 the rate of locomotion, as well as the precision of orientation, is 

 dependent upon the relative stimulating efficiency (or physio- 

 logical intensity) of the lights, those of greater efficiency causing 



