SIZE AND FORM PERCEPTION 



69 



other. On the end of DO is glued a piece of piano felt, P, which 

 rubs snugly against the shifter and prevents any intermingling 

 of the stimulus illuminations, while at the same time, the shifter 

 can be freely moved in either direction without becoming 

 scratched or marred by the friction. (4) N and N' are nest 

 boxes, 40^x22x22. Each nest box is covered by a tightly 



Figure 2. Experiment box 



fitting lid, which is hinged at the outside, and is equipped with 

 a 2 c.p. electric lamp (frosted globe), L, a watch glass for water, 

 G, and sand or litter in which food may be scattered. It is also 

 provided on the outer side with hidden holes for ventilation. 



Between W and N, also W and N', is a vertically sliding 

 door, E closed and E' open, 3 mm. thick which fills an opening 

 8x10. Suspended from an upright frame, F, is a coiled spring, 

 S, which passes through the walls of N' and B-W to the top 



