210 WILLIAM BRODBECK HERMS 



a vertical white screen standing at an angle of 45° to the ground 

 glass. Placing a photometer midway between the two areas the 

 lights were balanced by manipulating the sliding shutter. The 

 area of the smaller light after balancing was 7 sq. mm., while the 

 area of the larger field (the illuminated ground glass) was 52,800 

 sq. mm., or a ratio of approximately 1:7500. The intensity of 

 each source was 12 cp. 



The first set of experiments was tried on the migrated larvae. 

 The individuals were placed midway between the two lights, and 

 in all cases they took a straight path without turning toward 

 either Hght. This behavior would be expected under light of equal 

 intensities and at equal distances acting bilaterally upon these 

 organisms. It must therefore be concluded that the size of the 

 luminous area has no influence on the movements of the larvae. 



To test the adults a glass cylinder 20 cm. in diameter and 25 

 cm. in height was placed between the two lights so that the axis 

 of the cylinder coincided with the region of equal light intensity. 

 A small glass vial with a rectangular stage of black cardboard 

 around its neck served as receptacle for the individual flies as they 

 were tried. After transferring the fly from its original recepta-jle 

 into the vial, the latter was placed inside the glass cylinder, tilted 

 so as to rest on the edge of the stage with the longitudinal axis 

 of the vial on the line of equal light intensity. The cylinder was 

 then covered with a sheet of black cardboard. The flies, which are 

 negatively geotropic, naturally crawled up the vial and emerged 

 with the light from either side striking the two eyes equally. 

 Under these conditions fairly accurate results might be expected. 

 On protruding the head from the mouth of the vial the fly usually 

 paused for a moment, then either crawled out upon the stage or 

 flew immediately to one side or the other of the cylinder. Less 

 often it crawled the entire distance. In transferring the individuals 

 from one vial to another their negative geotropism and positive 

 phototropism were made serviceable. A little practice and pre- 

 caution were necessary to recapture the individuals after Hbera- 

 tion in the cylinder without crushing or losing them. 



Ten specimens of each species were exposed first to the two 

 areas separately and then to the two simultaneously. 



