SIZE AND FORM PERCEPTION 75 



The experimenter is thus enabled to control the intensities of 

 the stimulus areas without leaving his end of the apparatus. 

 By moving a carriage backward the intensity of the correspond- 

 ing stimulus display is decreased ; by drawing the source nearer, 

 the intensity is increased. 



Figure i also shows ■ arranged on a table, T, the electrical 

 connections with the experiment box. X, Y, and Z are 

 respectively an inductorium, a Columbia dry cell (No. 6), and 

 a telegraph key which, in connection with the wired slate floors 

 of W and W, constitute an interrupted circuit. The wires 

 are wrapped alternately about the slate floors so that by closing 

 the key, Z, a chick, when its feet touch any two of the wires, 

 can be shocked. With young chicks under two weeks of age 

 it was found necessary in general to set the secondary coil at 

 5, but as they become older and heavier the strength of the 

 shock had to be decreased, until, in some cases, the position of 

 the secondary coil was as far out as 6.5. The letters 5 and s' 

 indicate electric switches by means of which the illumination 

 in N and N' can be turned on or off. An upper illumination, 

 L, designed to hide inequalities in the distribution of light in 

 each compartment, W and W, hangs about 120 cm. above 

 the floor and directly over the center of the electric boxes. 

 It consists of a 2 c. p. electric lamp inclosed in a vertically sus- 

 pended cylinder of galvanized iron. The diameter of this cylinder 

 is 10 cm. and its length is 20 cm. The upper end is closed. The 

 lower end is fitted with a cover (like that of a baking powder 

 can) which may be pushed on or off. Centered in this cover 

 is an Aubert diaphragm by means of which the amount of light 

 falling on the experiment box may be regulated. On account 

 of the low ceiling of the dark room where these experiments 

 were conducted, I found it impracticable to use the diaphragm. 

 Unless it can be raised to a sufficient height it will cast shadows 

 upon the floor of the experiment box. With the diaphragm 

 removed, the upper illumination consisted of the rays from a 

 2 c.p. lamp passing through a circular opening 10 cm. in diam- 

 eter and falling a distance of 120 cm. 



The importance of L will be understood more thoroughly 

 in connection with the conditions for size discrimination. If 

 a chick is being trained to choose o 2 8+, 3 (a circle whose area 



3 The dimensions of stimulus areas refer in square centimeters to the area. 



