in which it was not shocked) 59 times in 100.
In other words the percentage of error was 41 instead
of 50. It is evident, therefore, that as low a
percentage of errors as 40 is not necessarily indicative
of discrimination. Anything below 40 per cent
is likely, however, to be the result of ability to
distinguish the brighter from the darker box.
To be on the safe side we may agree to consider 25
wrong choices per 100 as indicative of a just perceivable
difference in illumination. Fewer mistakes we
shall consider indicative of a difference in illumination
which is readily perceivable, and more as indicative
of a difference which the mouse cannot detect.
The reader will bear in mind as he examines Table
14 that 25 per cent of wrong choices indicates the
point of just perceivable difference in brightness.
RESULTS OF WEBER’S LAW EXPERIMENTS
Brightness vision
DATE NUMBER STANDARD VARIABLE DIFFERENCE %
OF ERRORS
OF TESTS
LIGHT LIGHT
May 13 100 20 9.4 .53
20
15 100 20 12.8
.36 36
16 100 20 10.8
.46 26
20 50 80 37.6
.53 6
21 50 80 51.3
.36 10
22 100 80 71.1
.11 35
24 100 80 60.0
.25 21
25 100 80 65.0
.19 25
27 100 80 80
0 41
28 50 5 2.5
.50 18
29 50 5 4.0
.20 14
29 100 5 4.5
.10 25
31 50 5 4.25
.15 20
June 1 50 5 4.85 .03
48
2 50 20
15.0 .25 16
3 50 20
17.4 .13 22
3 100 20
18.0 .10 22
4 100 80
72.0 .10 18
5 100 5
4.5 .10 12
7 100 5
4.67 .067 46
8 50 80
74.67 .067 56
9 50 20
18.67 .067 44
If we apply this rule to the results of the first
tests, reported above, it appears that a standard
of 20 hefners was distinguished from a variable of
9.4 hefners (.53 difference), for the percentage of
errors was only 20. But in the case of a difference
of .36 in the illuminations lack of discrimination
is indicated by 36 per cent of errors. A difference
of .46 gave a frequency of error so close to the required
25 (26 per cent) that I accepted the result as a satisfactory
determination of the just perceivable difference for
the 20 hefner standard and proceeded to experiment
with another standard value.
