334 
IOWA ACADEMY OF SCIENCE 
with each other, and so placed before each person mat he could 
easily see the exact time without attracting anybody's, atten- 
tion. No one then would notice another put down an observa- 
tion as to change in illumination. These results were therefore 
independent of each other and were compared and summarized 
only after the test was complete. 
The subjects were asked to read each his own text book while 
the test was going on. Each person was supplied with paper and 
pencil and requested to make note of changes in the illumination 
and the exact time at which the change took place ; also whether 
it was an increase or decrease; whether it was much or little, 
and whether it was objectionable. The lamp was fed by a 120 
volt storage battery, and hence was constant excepting for the 
changes purposely introduced. 
The voltage was varied all the way from 115.6 to 68 volts by 
steps of 1-6 volt to 33 volts. The test was run in three install- 
ments of about twelve minutes each. The eyes had to be kept 
constantly on the book so as not to miss a change, and hence a 
much longer period was deemed undesirable. The changes were 
made at intervals varying from 30 seconds upward. 
It may be noticed from the curve Plate XXXVIII, that a 
change of less than one volt is noticed by practically nobody and 
that a change of five and one-half volts is noticed by practically 
everyone. Account was also taken of whether the voltage change 
was an increase or a decrease. As there was practically no dif- 
ference between the two in the effect upon vision the results 
were all plotted together in Plate XXXVIII, but it seems worthy 
of note that the eye is no more nor less sensitive to an increase in 
illumination than it is to a decrease. The subjects experimented 
on noticed every decrease as a decrease and vice versa. Such 
careful distinctions are perhaps not always made under ordi- 
nary conditions, for a person reading in his own home is not 
expecting changes in voltage as these subjects did. 
The curve in Plate XXXIX shows how great a change in volt- 
age is permissible without being objectionable. A change of 25 
volts was objectionable to practically everyone, and a change of 
11 volts was objectionable to half of the subjects. This test con- 
tinued for only a comparatively short time. It is reasonable to 
.suppose that the eye would show greater sensitiveness to changes 
and be more inclined to object if the voltage should show va- 
riations over a long period of time, say weeks or months. For 
