PHYSIOLOGY: LOEB AND NORTHROP 
543 
Each table is the average of from 40 to 60 individual observations, each 
being the average of the path of many thousands of animals. 
In the next series of experiments an opaque rotating disk with one 
sector cut out was placed before Kght B. In one set of experiments the 
sector cut out was 90°. The rate of rotation (by an electric motor) 
was 1,500 to 2,500 revolutions per minute. The other Hght was con- 
stant and its position was chosen on the assumption of the validity of 
the Bunsen-Roscoe law for these cases. Thus when the two lights 
without sector were equal at a given distance of ^, by putting the 90° 
sector before B, it was assumed that the ratio of effects would be the 
same as if, with constant light, B had been placed at the double dis- 
tance and the ratio of intensities of the two lights had been 4:1. Going 
on such a calculation we should expect the same values for a as in table 1 . 
As one sees the observed values are 
slightly smaller but practically identical j^^ ^ 
with the values obtained when the two 
lights are constant. The deviation is prob- 
ably due to the fact that the photochemical 
efficiency of an intermittent light is a trifle 
less than that calculated on the basis of 
the Bunsen-Roscoe law.^ 
We carried out some experiments with a 
sector of 144°. When the efficiency of both 
lights was equal on the assumption of the 
validity of the Bunsen-Roscoe law a was found to be 44.9° (instead of 
45°), and for the ratio 2:1a was found to be 38.8°. The values are, 
within the limits of error, identical with the values in tables 1 and 2. 
TABLE 1 
VALUE OF a. FOR DIFFERENT RATIOS OF INTENSITIES 
OF THE TWO LIGHTS 
Ratio of the two lights 
1: 1 
2: 1 
4: 1 
10: 1 
Value of a (direction of path) 
45.6° 
40° 
34.4° 
28.8° 
TABLE 2 
VALUE OF a. WHEN ONE LIGHT IS INTERMITTENT (90° SECTOR) 
AND THE OTHER CONSTANT AND THE EFFICIENCY OF THE 
TWO LIGHTS IS CALCULATED ON THE BASIS OF 1HE VALIDITY 
OF THE BUNSEN-ROSCOE PHOTOCHEMICAL LAW 
Ration of the two lights 
1: 1 
2: 1 
4: 1 
Value of a 
44.2° 
38.3° 
34.1° 
