CHANGE OF LAMP VOLTAGE 333 
THE EFFECT OF CHANCE OF LAMP VOLTAGE ON 
VISION. 
WM. KUNERTH. 
It must be apparent to any one who has tried to read or study 
by the light of an incandescent lamp that there is; occasionally 
such a great change in the impressed voltage as to be decidedly 
objectionable. These changes may be due to a turning on or off 
of a great number of lamps at the same instant, or to the closing 
or opening of a circuit containing a relatively big power con- 
suming agent. 
This report is to give an answer to the question as to how 
great a change in lamp voltage, is permissible without being 
objectionable to the eye, and how small a change in lamp voltage 
can be detected by a person reading. It is; based upon experi- 
mental results obtained from actual conditions in the laboratory. 
The persons experimented on (10 in number) were seated 
in such a way as to have the light fall over their shoulders. 
They were college students with normal eyesight. The room 
was illuminated by a 100-watt, 110 volt tungsten provided with 
a shade. The normal intensity of illumination at the various 
places where these subjects used light varied from 0.84 ft. can- 
dles to 1.44 ft. candles, but the variation in sensibility of the 
eye between these limits is very small as shown by the curve 
in Plate XXXVII taken from Wickenden ’s ‘ ^ Illumination and 
Photometry’’. This variation was not sufficient to cause any 
serious error, for no appreciable difference was noticed between 
the data of the person who had high illumination as compared 
with the person who had lower illumination. It was therefore 
assumed that all the data thus taken could be compared on prac- 
tically the same basis. 
A mercury rheostat and a voltmeter were in the lamp, circuit, 
and changes; in the rheostat could be made without having any- 
body in the room become aware of it except as he noticed a 
change in the illumination. Also the change in voltage was 
sudden, just as on a line when energy is turned on or off, for it 
was accomplished by merely putting in or picking up a link. 
The watches of all people in the room were adjusted to agree 
