14 'Journal of Cornparative Neurology and Psychology. 



was sometimes raised before the episcotister had gained full speed. 

 The flickering light never failed to frighten the animals. They 

 would never leave my shoulder to make a choice until the lights 

 appeared perfectly steady to my oirn eyes. 



Method of Determining Intensity of Ielumination of 



MONOCIIKOMATIC BaNDS. 



After vainly trying to obtain a photometric reading of the minimal 

 intensity of the monochromatic bands,i^ with a photometer based 

 upon the Joly principle, I finally, on the advice of Professor 

 Milliken, abandoned the photometer and had resort to the simple 

 apparatus, the groimd plan of which is shown in Fig. 5. In the 

 diagram, V is a band of monochromatic light visible upon the 

 ground glass surface. C is a white surface (bristol-board; plaster- 

 paris is preferable) equal in width to Y , which reflects light to 

 the prism, P, from a source the intensity, and distance from C of 

 which, is known. P is a 90° prism silvered on the two surfaces 

 which reflect the inuiges of V and C iuto the eye at E. The distances 

 from V to P and from C to P are equal, 15.5 cm. The distance 

 from P to -E' is 20 cm. The total distance from E to V approx- 

 imately equals the distance of the color from the eye of the monkey 

 when he reaches the partition GP in Fig. 2 and makes his choice as 

 evidenced by his going to the right or to the left of the partition (the 

 monkey B. often stoj^ped at this j)oint and turned his head first to 

 the right, then to the left, etc., before finally making his choice). 



The photometric determinations were made in a dark-room under 

 conditions as nearly as possible like those under which the animal 

 reacts. The eye was dark-adapted (15 to 30 minutes). A com- 

 fortable position was taken with the eye at E so that a clear reflected 

 image of V appeared. An assistant then lighted a standard electric 

 light which was screened from the observer's eye by the opaque 

 screen, 08, and mounted it upon the board, B, which was graduated 

 in cm. The distance of this light was varied until the observer at 

 E judged the two lights to be equal. The judgments made under 



"They are not really sources iu the technical sense, but surfaces. 



