Reactions of I so pods to Light 277 



intensity was used. It was placed at the distance of one meter 

 from the nearer end of the outer tank. The photometer was 

 placed close to the opposite end of the outer tank, so that the light 

 from the lamp had to traverse the water in both tanks. The 

 luminosity at the photometer ( 1.72 meters from the lamp) was 

 measured by using as a standard of comparison a single-glower 

 Nernst lamp whose candle power had been previously determined. 

 In like manner was determined theluminosityat the photometerat 

 the same distance after removal ot the tanks. On the basis of 

 these determinations the water used in the tanks in these experi- 

 ments was found to cut down the light passing through it at the 

 average rate of ^ per cent for each centimeter. By calculation, 

 the luminosity at each part of the tank was then determined. 

 The luminosity at the middle of imaginary section number i, the 

 section nearest the lamp, was found to be 620 CM., that at Sec- 

 tion 6, 251 CM., or a little more than two-fifths that at Section i. 

 When, however, the lamp was placed at a distance of only 20 cm. 

 from the outer tank, the distance at which it often was used in 

 the experiments, the difference m luminosity at the two ends of 

 the tank was still more. 



A computation based on the above determination showed that 

 when the six-glower Nernst lamp was used at 20 cm. from the 

 near end of the outer tank the luminosity at the middle of Section 

 I was 7863 CM., whereas at section 6 it was only 1065 CM. or less 

 than one-seventh as great as at Section i. 



These determinations, while they can be only approximate, 

 show that the difference in luminosity between the two ends of 

 the tank (and the difference would of course be greater the nearer 

 the lamp was to the tank) was very considerable. In order to 

 obtain high intensities of illumination the lamp was, in most of 

 the experiments, used very near to the tank; hence the difference 

 in luminosity between Sections i and 6 was usually very great. 

 Since the difference in illumination between the two ends of the 

 tank was so great, there is sufficient basis in that fact for explain- 

 ing the collection of Caecidotea in the negative end of the tank as 

 the result of photokinesis alone. 



Photokinetic movements were very marked with Cecidotea 



