436 PHOTOREACTIONS OF WHIP-TAIL SCORPIONS 



were recorded in terms of deflection from an initial direction of loco- 

 motion. Deflection amplitudes were measured in degrees by record- 

 ing the animal's point of emergence from a graduated circle. By 

 making the diameter of the circle relatively small (50 cm.), the rate of 

 attaining orientation, as well as the accuracy of orientation, was m.ade 

 measureable. 



The accuracy of orientation exhibited by normal animals when 

 placed between equal, opposed beams of light, was measured as the 

 reaction which would be most readily disturbed by any asymmetrical 

 interference with the receptive mechanism. Deflection amplitudes of 

 animals subjected to anterior and to lateral illumination were meas- 

 ured as being the reactions which would be most extensively changed by 

 symmetrical eUminations of receptors. 



The experiments on partially blinded animals reported below were 

 carried out with the same intensity of illumination, and the same 

 method of handhng used on normal animals. Change from the nor- 

 mal reaction following the elimination of a photoreceptor can, there- 

 fore, be taken as a measurable index of the effectiveness of the receptor 

 prevented from functioning. A series of experiments in which all 

 possible cases of elimination are carried out will show at the same 

 time the relative effectiveness of the different members of the receptive 

 system, and the effect on orientation produced by symmetrical and 

 by asymmetrical interference with the photoreceptors. 



Apparatus. 



The apparatus used in these experiments was the same as that used 

 in measuring the reactions of normal animals. It is shown in Fig. 1. 



Determinations of the illumination delivered by the lights were 

 made with a Bunsen photometer. Each fixed light gave an illumina- 

 tion of 120 candle meters at the center of the observation circle. The 

 intensity of the third light could be varied by moving it to different 

 positions along the axis. The whole apparatus was located in a dark 

 room and in order to reduce reflected Hght to a minimum, all parts of 

 it except the scales were painted flat black. 



