METHODS OF STUDYING VISION IN ANIMALS 55 



The extreme positions of carriage H are adjustable by means 

 of a stop, 71, on the left-hand side, and by a similar stop on the 

 right-hand side, which is hidden by the carriage. The extreme 

 positions of block 46 are likewise limited by stops 72 and 73. 

 When carriage H hits its left-hand stop, 71, block 46 hits its 

 left-hand stop, 72. A similar condition obtains for the right- 

 hand position of both. 



In the center of block 46 is motmted a circular table, 74, free 

 to rotate in a horizontal plane. On a diameter of this table 

 and 4" above it is mounted by means of two L-shaped blocks, 

 75, a metal track, 76, which carries the two episcotisters, P and 

 K. The distance between centers of these episcotisters can be 

 adjusted by clamping them at any desired position on the track. 

 They are belted by means of small round leather belts or rubber 

 bands to a small motor, 77, screwed firmly to the circular table 

 74. The speed of the motor can be controlled by the rheostat, 



78, which is also screwed to the table. The wiring to the motor 

 and rheostat is carried to the table through a V' brass tube, 



79, which serves to keep the wires from interfering with the 

 beams when the horizontal table is being revolved in order to 

 change the relation of the episcotisters. The rotation of the 

 circular table, and thus of the episcotisters, can be controlled 

 by the operator at G, by means of a continuous cord system 

 passing around a circular drum lying between block 46 and 

 table 74, to which it is concentrically attached (not shown in 

 this cut, but separately in figs. 11 and 12.) The cord system 

 may be traced as follows : The gut cord 80 passes over a pulley, 

 81, to iron rod 82, to turn-buckle 83, to gut cord 84, to pulley 

 85, to iron rod 86, through a clamp 87, to another gut cord 

 which runs over a pulley to a turn-buckle not shown, to iron 

 rod 88, to gut cord 87, over pulley not shown, back to the drum. 



a. Operation of the system' — The purpose of these various 

 pulley systems is to make the shifting of the colors and the 

 control of their intensity absolutely automatic. The operator 

 at G can reverse the colors by merely pushing the carriage to 

 the right or left for a distance of 25 cm. At the same time, 

 this movement of the carriage can be made to control the posi- 

 tions of the episcotisters. With the carriage at its extreme 

 right position the green is on the right and the red on the left. 



