I907.] TITCHENER AND PYLE— JUDGMENT OF DISTANCE. 97 



The shadows were cast by angular pieces of stiff black cardboard 

 (angular magnitude, 60° ; length and width of legs, 45 and 5 mm. 

 respectively). One of these was fastened to the back of the sheet, 

 its vertex coinciding with the centre of the central vertical line on 

 the front. The other two moved in or out with movement of the 

 limiting strips. The arrangement was simple : the rods carrying 

 the strips w^re bent round, to the back of the frame, and there 

 passed through guides placed at the level of the front guides and 

 strictly parallel to them. The cardboard angles were attached to 

 the inner ends of this second pair of rods, with their vertices at the 

 centres of the strips. Whenever, then, the limiting strips were 

 moved, their angles moved with them, and strip in front and angle 

 behind maintained always the same relative position. 



The apparatus thus constructed was set up on a table in a dark 

 room. Its front surface was illuminated by two hooded incandescent 

 lamps, placed symmetrically on either side and at equal distances 

 from the sheet ; its back surface was illuminated by a single hooded 

 lamp, placed opposite the centre of the sheet. This third lamp was 

 controlled by a rheostat, and all three were on the same electric 

 circuit. The bulbs were of ground glass, and the light of the lamps 

 was further diffused by sheets of tissue paper. 



The procedure was now as follows. The observer, seated before 

 the apparatus, was left in the dark for 15 minutes, in order that 

 his eyes might be properly adapted for the experiments. The 

 lights were then turned on, and the experimenter held up, directly 

 behind the frame, a circle or a skeleton square cut from black card- 

 board. The intensity of the light at the back was slowly reduced, 

 until the observer was just unable to detect the shadows cast by 

 these figures. Or, rather, the reduction of the light was arrested 

 at a point somewhat short of this : at the point, namely, when the 

 observer declared that there might perhaps be a shadow there, on 

 the white background, but that he could not possibly decide whether 

 it was the shadow of a square or of a circle. We may say at once 

 that the arrest of the rheostat at this point caused us some incon- 

 venience in the experiments proper, since it not infrequently hap- 

 pened that the observer remarked, in the course of a series, that 

 he thought he saw a shadow on the white sheet : in which case the 



PROC. AMER. PHIL. SOC, XLVI. 185G, PRINTED JULY I5, I907. 



