258 BRADLEY M. PATTEN 



for animals with some part of the receptive apparatus ehminated 

 bilaterally, it should be possible to deduce with reasonable 

 accuracy the part played in orientation by the eliminated 

 photoreceptors. 



We know also that a negative animal will continue to crawl 

 away from a source of light when headed in that direction, and 

 that when subjected to equal opposed lights, it will move along 

 a norm to the line connecting the two sources. These reactions 

 both result from the tendency of a negative animal to maintaia 

 stimulation in bilateral equilibrium. The accuracy of the re- 

 actions depends on the sensitiveness, and especially on the 

 bilateral balance in effectiveness, of the receptors. By interfer- 

 ing with the photoceptors of one side only and comparing with 

 the normal, the measurements of the unbalanced reactions thus 

 induced, it should be possible to obtain additional evidence as to 

 the relative importance of any particular part of the receptive 

 mechanism. 



We should, therefore, be in possession of the desired data on 

 which to base further work, if, by the use of some reliable quan- 

 titative method (such as measuring the angular deflection from 

 an original path of locomotion), we determined the normal re- 

 actions, (1) to a single horizontal beam of light of known inten- 

 sity acting laterally on the animal; (2) to a horizontal light 

 acting on the animal from in front; (3) to a horizontal light 

 acting on the animal from behind ; and (4) to equal opposed lights 

 acting on the animal bilaterally. 



The measurements first made, were obtained by subjecting 

 animals to a single horizontal beam of light from the side. The 

 illumination at the center of the observation circle was 120 

 candle meters. As a precaution against the possible distorting 

 effect of unbalanced sensitiveness, the animals were placed at the 

 center of the observation circle heading across the beam first one 

 way, then the other, so that right and left sides were subjected 

 alternately to the light. (For discussion of this method see 

 Patten, B. M. '14, pp. 230-233.) Although no visual response 

 had been indicated, the fights A and B (fig. 2) were each used 

 for half of the measurements so that the animals should be crawl- 



