1 4 TH E M I N D O F TH E BUY E R
line which ap pea rs to you to equal the horizontal line.
( D o not try t o make allowance for the discrepancy which
you no w know ex ists. )
I f you measure the line you have j ust drawn yo u will
p robably find that it is less than fou r inche s ; thus showing
that you j udged the line longer when in the vertical p osi
tion, than in the ho riz ontal. A nd you can state h ow much
longer you j ud ge d it by subtracting the length of you r
Copied line from the length o f the standard. But this single
observation is no t sufficient to permit a scientific conclusion.
In scientific investiga tion a single observation is seldom relied
upon. Y o u must make more measu res. Co ver up the line
yo u j ust drew and draw another ; cover it up and draw
successive lines, covering each line as so on as drawn, until
you have d rawn ten.
Now measu re all the lines and record the lengths ; a dd
them and find the avera ge. Y our series o f measu res will
resemble the se ries below, showing the lengths o f lines drawn
by anothe r ex perimente r under these same conditions. The
measu re s are stated in centimete rs. With a horizontal line
o f ten centimete rs ( fou r inches) as a standard the vertical
line was drawn ten times with the following lengths
The average sho ws a di fference o f
centimeters between the horizontal standard
and the vertical copies. This gives us sound
basis for concluding that under the condi
tions o f the expe rime nt, the appa rent length
o f a line ch anges from 1 0 centimeters to
centimeters when the line is changed
from horizontal to vertical.