270 BRADLEY M. PATTEN 



These reactions serve to establish clearly the sign and the 

 general nature of the reactions of the whip-tail scorpion to 

 light, but they do not serve to give us any information con- 

 cerning the mechanism of orientation not already available 

 from the recorded reactions of other forms. 



The responses of normal and partially blinded animals to 

 balanced opposed hghts, however, bring out certain points of 

 interest which are not so apparent in experiments with a single 

 beam of light. The response of normal animals under equal 

 bilateral illumination was locomotion in a direction at right 

 angles to a line connecting the two sources of light. In these 

 reactions there appeared no semblance of anything which could 

 be interpreted as a 'trial movement.' There w^ere no side to 

 side swingings which would produce a rapid sequence of changes 

 of intensity on the photoreceptors. The animals moved be- 

 tween balanced lights with more directness and precision than 

 in any of the other experunents. Under such conditions the 

 distribution of the stimulus on the photoreceptors is bilaterally 

 symmetrical, and the effective intensity is relatively constant. 

 Is the symmetrical distribution of the stimulus merely incidental 

 to an orientation governed by "other factors, or is it a critical 

 factor in the maintenance of orientation? The question may 

 best be answered by interfering with the symmetrical stimulation 

 and ascertaining the effect on orientation. In table 5 are given 

 reaction measurements obtained by subjecting animals in which 

 one median eye had been covered, to equal bilateral illumination. 

 If symmetrical stimulation is incidental, we should expect to 

 find no change from the normal reactions to the same conditions 

 of illumination. A comparison of the reactions recorded in 

 table 5 with the normal reactions (table 4) shows that bhnd- 

 ing one median eye produced consistently a deflection toward 

 the blackened eye. Table 6 records a similar series of measure- 

 ments made on animals in which one of the lateral eye groups 

 had been covered. The results are in complete agreement 

 with those obtained by covering one median eye. Further- 

 more it was demonstrated experimentally that a similar un- 



