Mast, Light Reactions in Lower Organisms. I2I 



the aquarium it deflected to the right 17 degrees, on its second 

 trip 15 degrees, and on its third trip 11.5 degrees. The following 

 thirty trips were made with so little deviation from 1 1 degrees 

 that it could not be measured. The experiment was closed at 

 10.45 ^- "^-j ^^o hours and fifteen minutes alter it was begun. 



It appears, then, that when the internal factors have become 

 stable, and the external factors are not changed, the angle of deflec- 

 tion remains constant. 



b. Effect of Change in Light Intensity on Orientation. 



In general a decided increase or decrease in light intensity 

 causes an increase in deflection. This seems to be connected 

 with the fact, pointed out by Holmes ('03, p. 321), that in low 

 or high light intensity the colonies are not strongly positive. 



On August 3, 1905, the relation between the course of a given 

 colony and the ray direction was obtained in a light intensity of 

 400 candle meters and also in an intensity of 20 candle meters. 

 In the higher intensity, the deflection to the left was found to 

 be I degree; in the low^r intensity 11 degrees. The course was 

 ascertained by letting the colony across the aquarium three 

 times in succession in the lower intensity, then three times in the 

 higher, then twice in the lower, and finally twice in the higher. 

 The light intensity was reduced by cutting off part of the light 

 with a screen, which contained a narrow slit, placed close to the 

 Nernst glower, and so arranged that the slit was perpendicular 

 to the glower. Neither the light nor the aquarium had to be 

 touched in decreasing or increasing the light intensity, so the ray 

 direction was unquestionably the same under both conditions. 

 There was remarkably little variation in the angle of deflection in 

 all the trips made across the aquarium in either light intensity. 

 There can thus be no question about the accuracy of these observ- 

 ations. This experiment was repeated a few days later with 

 similar results. The colony selected, however, deflected to the 

 right instead of to the left, as the one in the first experiment had 

 done. The deflection in the second experiment was studied in 

 three diff"erent light intensities: 20 candle meters, 400 candle 

 meters and nearly 2000 candle meters. The highest intensity was 

 produced by a carbon arc. The angle between the light rays and 

 the course taken by the colony was found to be 12 degrees in 20 

 candle meters intensity; 2 degrees in 400, and 40 degrees in 2000. 



