144 
IOWA ACADEMY OF SCIENCE Voe. XXIX, 1922 
After the twenty-six individuals had been under any one con- 
dition of illumination for upward of five minutes they were asked 
to put down their opinion of the lighting on the basis of instruc- 
tions given them. Then the current through the lamps was 
changed and observations again were made. In this way five dif- 
ferent values of current were' used, changes in intensity being made 
by irregular stages and not always in the same direction ; and the 
individuals did not know but that any value of current might be 
duplicated. Then the other system was installed and the five 
conditions were repeated, except that the current values differed 
from the first set because of the difference in the efficiency of in- 
stallation already noted above. During the few minutes while 
the change was being effected from one system to the other, the 
students were free to move about in the room. The arm of the 
chair on which the book was placed was 28 inches above the floor 
and sloped at an angle of 2 y 2 degrees from the horizontal. 
After the individuals had recorded their estimates of the illu- 
mination on their books under these conditions, the same condi- 
tions were repeated and the intensity in foot-candles which each 
individual had had on his book was measured by means of a 
Macbeth illuminometer. The number of foot-candles which each 
one considered as “just right” by the two systems is given in the 
table herewith. It may be noticed that a few estimates are very 
high, while a few others are very low. 
INTENSITIES IN FOOT-CANDLES REQUIRED FOR 
READING ORDINARY TEXT BOOK PRINT 
BY TWO SYSTEMS OF LIGHTING 
Direct 
Indirect 
Direct 
Indirect 
1 
2.0 
2.5 
15 
3.0 
3.0 
2 
7.0 
5.7 
16 
2.3 
5.0 
3 
3.0 
2.0 
17 
3.0 
6.5 
4 
2.3 
2.3 
18 
4.0 
6.7 
5 
1.1 
1.9 
19 
3.0 
3.0 
6 
1.8 
4.9 
20 
3.0 
3.0 
7 
2.0 
6.0 
21 
1.0 
3.7 
8 
5.0 
6.5 
22 
1.5 
6.0 
9 
5.0 
3.0 
23 
2.0 
6.0 
10 
3.0 
5.0 
24 
2.3 
2.3 
11 
2.5 
4.3 
25 
2.3 
2.5 
12 
3.0 
2.3 
26 
1.7 
3.0 
13 
3.0 
4.5 
Ave. 
2.8 
4.0 
14 
3.0 
3.5' 
Even though some required much less illumination intensity 
than is ordinarily considered satisfactory, it may be noted that 
they required less of both indirect and direct and hence the ratio 
