BUNSEN-ROSCOE LAW 89 



could show that when the duration of illumination of one 

 eye was altered by a rotating opaque disk with one sector 

 cut out, the heliotropic effect on the eye of Daphnia was 

 the same as when the intensity i of the same light was 

 reduced to a.n amount corresponding to the Bunsen- 

 Boscoe law. 



Under the influence of two constant lights of equal 

 intensity heliotropic animals move in a direction at right 

 angles to the line connecting the two lights. If the law of 

 Bunsen and Roscoe holds the effect of a constant light 

 should be diminished if a rapidly rotating opaque disk 

 with one sector cut out be put in front of the light, and the 

 diminution should be equal to the fraction of the arc of 

 the sector. Thus a sector of 90, which reduces the total 

 duration of illumination to one-fourth, should also reduce 

 the heliotropic effect of the light to one-fourth, and the ani- 

 mal should deviate from the old direction in the direction 

 toward the light without a disk before it. If, however, we 

 lower the intensity of the latter light to one-fourth by 

 doubling its distance we also reduce its heliotropic effect 

 to one-fourth, and now the animal should move again in a 

 line at right angles to the line connecting the two lights. 



The following experiments carried out by Loeb and 

 Northrop 309 on the larvse of the barnacle are perhaps the 

 best proof for the validity of the Bunsen-Eoscoe law for 

 animal heliotropism. 



These animals are small and can be obtained in large numbers. They 

 were made to collect in the corner of a dish with a little sea water and 

 were then sucked up into a pipette ef, Fig. 31, which was blackened with 

 the exception of the opening. When such a pipette is put into a glass 

 dish with parallel walls whose bottom is black (by putting paraffin black- 

 ened with lampblack at the bottom of the dish) the larvae will flow out in 

 a fine stream and swim when they are positively heliotropic in a straight 

 line toward the source of light. They thus form a rather narrow white 

 trail on the dark bottom and it is possible to measure the angle of this 



