28 ROBERT M. YERKES 



the one of the stimulus areas presented to the subject was always 

 illuminated, whereas the other was entirely unilluminated, except 

 as general illumination was employed in the experiments. Natur- 

 ally, as the experiment-box was shifted from side to side, the 

 more intense achromatic stimulus was presented now in the 

 stimulus compartment on the right of the subject, now in the 

 one on the left. 



The writer is convinced that wherever possible the inter- 

 ference of the experimenter in the course of an animal's reaction 

 should be obviated by the use of automatic or subject-actuated 

 devices. It was not feasible, however, in the present investiga- 

 tion, to introduce such devices, — consequently the use of the 

 slide-doors, as shown in figure 1. These, it should be stated, 

 proved surprisingly satisfactory in the case of the ring-dove, 

 which is easily startled and which would not react well in a 

 subject-actuated apparatus unless everything could be made to 

 work steadily, quietly, and fairly slowly. It is, however, beyond 

 question that our efforts in studies of behavior should be to 

 eliminate, as far as possible, the necessity, during the course of 

 reaction, of movements by the experimenter which tend to 

 modify the behavior of the subject. It has repeatedly appeared 

 that even the experienced investigator is liable, unconsciously, 

 to supply cues to his subject which facilitate proper reaction, 

 or even serve as the sole basis for what appears to be discrimi- 

 nation. 



The birds used for the present work were obtained from a 

 Boston dealer. All that could be learned about them was that 

 they were young We are therefore under the disadvantage 

 of being unable to give a satisfactory description. It is 

 obviously desirable in all such investigations that the origin 

 and exact age, as well as the sex and history of each subject, 

 should be known. But this is somewhat less essential, it must 

 be admitted, in the case of preliminary observations than in 

 that of continuation-work. Four birds were used. Of these, 

 two, supplied as male and female by the dealer, in reality both 

 males, appear as numbers 1 and 2 in this report. They were 

 used over a period of several weeks by Mr. A. M. Eisenberg. 

 The others appear as number 3, a female, and number 4, a male. 

 During a period of five months these birds were used in the 

 visual experiment by the writer. The results obtained with 



