86 ROBERT M. YERKES AND JOHN B. WATSON 



the partition. These three precautions totally eliminate all 

 difference in the behavior of the long and short wave-lengths, 

 for the very simple reason that all the reflecting surfaces where 

 light immediately falls have been removed. 



(a). The diaphragms are made from 1-32" sheet brass. Eight 

 pieces 4" x 7'' are cut out, and in the center of each a rectangular 

 opening 2" x 4" is milled and knife-edged. Four such dia- 

 phragms should be mounted together and placed in front of 

 each of the two windows in the experiment box. They should 

 be mounted as follows: The four 4" x ^" sheets are clamped 

 together and drilled at the four corners with a \" hole; four 

 \" brass rods are cut, 6f" long and are tapped and supplied 

 with hexagonal nuts \" thick. The four diaphragms are slipped 

 on these rods, and are separated from one another by means of col- 

 lars 2" long, made from brass tubing, the inside diameter of 

 which is \" . The set of diaphragms is acid-blackened. This 

 method of mounting the diaphragms enables one to dispense 

 with a supporting floor, which would offer a reflecting surface. 

 The diaphragms when assembled should be mounted in front 

 of the two windows in a bearing which offers universal adjust- 

 ment, for ease in centering with respect to light. (As a pre- 

 cautionary measure it is well to cover each of the diaphragms 

 with dead black velvet. The velvet should not be placed nearer 

 than Y' to the edges of the knife edged rectangular opening.) 



(b). The non-reflecting floor grill is made to serve both as a 

 punishment device (if desired) and as a means of absorbing 

 light. It is made as follows: (The size given is one fitting the 

 floor of an experiment box made for small mammals. This 

 box is similar to the one shown in fig. 4. The grill fits snugly 

 the whole of compartment B.) Three \" brass rods 20^ long 

 are tapped at their ends and supplied with nuts \" thick. The 

 whole rod is bushed with wood-fiber tubing, the outside dia- 

 meter of which is f". Forty strips of brass 1-32" thick, \" 

 wide and 20" long, are then drilled at the center and at the 

 two ends with a 7-16" hole, so as to admit the three bushed 

 rods with a good deal of play. The strips are all acid blackened 

 and slipped on the rods one at a time. They are inclined at 

 an angle of 60°. A fiber bushing \" long, sawed at an angle of 

 60° is slipped on each of the rods between each of the metal 

 strips. Before blackening, the strips should be carefully smoothed 



