Watson, Color Vision in Monkeys. 13 



of 30° was cboseii as the miuimum after several preliminary trials. 

 It was desirable to keep the minimum intensity of any beam always 

 well over the human threshold. Any smaller angle did not permit 

 this, wath sunlight as the source. That this minimum w^as also well 

 above the animal's (reaction) threshold was tested in the following 

 way: The episcotister, set at the minimum (30 '), was allowed to 

 interrupt the blue ; the yellow was cut out at ^'o and the animal 

 tested at X in the ordinary way. When the screen was raised, only 

 the blue band appeared. As a result, it was found in every case 

 that the animal followed the light regardless of its right or left posi- 

 tion. The blue was then cut out at >S'2 and the yellow interrupted, 

 with similar results. There is the possibility, however, that the 

 minimum intensity was over the 'brightness limen' but not over 

 the 'color limen.' This objection cannot be met until extended 

 threshold tests have been made. 



In conducting such experiments in the future, the following pro- 

 cedure will be adopted: first, during the formation of the associa- 

 tion, an episcotister opened to the maximum (320°) will interrupt 

 each beam continuously during all tests ;^^ second, after the associa- 

 tion has been established, the control tests will be made with the 

 two episcotisters set at any desired angle ; third, the iris diaphragm 

 will be used as an additional control. ^^ 



The episcotister was run at a very high rate of speed. As a 

 test as to whether the beams were uniform for the animal, the screen 



"In duplicating the apparatus at Hopkins, the motor and the two episcotisters 

 were mounted upon a small revolving table. This table is so arranged that a 

 pull on a cord (at X, Fig. 2, where the experimenter sits) will interchange 

 the positions of the two episcotisters, thus making it possible to have the 

 animal on the one trial react, e. g., to "minimum red," "maximum gi'een," 

 and on the next to "maximum red," "minimum green." 



"It will be remembered that there are three common ways in which intensity 

 in the physiological sense can be altered: (1) by decreasing the amplitude 

 of the ether waves of the beam which falls upon a given retinal area (increas- 

 ing distance of source) ; (2) by lessening density of beam (use of diaphragms, 

 etc.) ; (3) by interrupting beam (episcotister). In order to test whether the 

 physiological effect, e. </., of distancing the source of the beam is the same 

 as interrupting it, it is desirable to make changes in intensity by employing 

 all three methods. The desirability of the use of the episcotister and dia- 

 phragm has been assumed in the present work. 



