Do our perceptions of size 
depend on such stimulus 
relations? In an attempt to 
answer this question, Sheldon 
Ebenholtz and I carried out the 
following two-part experiment. 
In one part, an observer sat 
midway between two luminous 
vertical lines in an otherwise 
dark room. One line, the 
standard, was 3 inches high, 
and the other was a line whose 
length could be varied by the 
subject. The observer was asked 
to look back and forth and to 
indicate when the length of 
the variable line matched the 
standard. In this, the control 
part of the experiment, subjects 
were able to match the standard 
line’s length almost exactly, 
selecting on average a line 3.1 
inches long.

    In the second part of the 
experiment, the observer was 
again asked to match the lines, 
but this time they were 
surrounded by luminous 
rectangles—–actual frames of 
reference. The standard line 
was enclosed in a rectangle 4 
inches high, making it three-
fourths the height of the 
rectangle, and the variable line 
was enclosed in a rectangle 
three times larger, or 12 inches 
high.