56 ROBERT M. YERKES AND JOHN B. WATSON 



Episcotister K intercepts the green beam, while P intercepts 

 the red. Shifting the carriage 25 cm. to the left, brings the 

 green beam to the left and the red to the right, but episcotister 

 P now intercepts the green, while K intercepts the red. Some- 

 times this is desirable. But suppose we desire to shift the posi- 

 tion of the color without changing the relation of the episco- 

 tisters; one needs then merely to clamp the cord system con- 

 trolling the circular table at 87. This will cause the circular 

 table carrying the episcotisters to rotate 180° while the carriage 

 H goes from one extreme position to the other. P will thus 

 be made always to intercept the red, and K the green beam. 

 It is desirable at still other times to use the full intensity of 

 the beam. This is accomplished by first rotating the episco- 

 tister table 90° from the position shown in the drawing, and by 

 releasing the clamp screw at 87. The episcotisters will no longer 

 intercept either of the beams in either position of carriage H 

 (see, for mechanism of episcotister table, p 66). 



A somewhat more expensive but more satisfactory substitute 

 for carriage H is a rectangular dark cabinet, fig. 8, 40'^ high. 

 20" thick by 40" wide. The vertical framework of the cabinet 

 is made of four pieces of oak, 3" x 3" x 40", bolted together by 

 heavy iron straps |" thick, 3" wide and 40" long. The sides and 

 top are covered with thin wall-board, coated with dead black 

 paint. The inside of the cabinet is lined with dead black velvet, 

 loosely glued to the sides. The three speculum mirrors, M, Mj 

 and M2, are fastened by means of their brass framework to the 

 strap Ba, 25 cm. between centers. The mirrors are inclined 

 as before, at an angle of 45° to the incident beam. Below each 

 mirror stands a metal box containing at its upper end the sur- 

 face of opal glass or plaster of paris, upon which the beam com- 

 ing from the mirror is to fall. The beams are admitted through 

 the three windows, W, W^ and Wj. The arrows show the courze 

 of the beams, G, Rd and Gj. 



There are two sets of these metal boxes: one set of three, 

 which permits the use of a plaster of paris surface, and another 

 of three, of opal glass. For convenience we shall call thess 

 respectively " plaster of paris boxes " and " saturation boxes." 

 The plaster of paris boxes are used in all cases where it is desired 

 to give a stimulation of monochromatic light at maximum 



