METHODS OF DETERMINING TOLERANCE. 37 
The “ normal paper ” is prepared in a very simple way. Ordinary 
paper used for photographic purposes is soaked in a 3 per cent solu- 
tion of common salt, and then hung up to dry. After the paper has 
become thoroughly dry it is immersed for two minutes, in the ab- 
sence of chemically active rays, in a 12 per cent solution of silver 
nitrate. After this the paper is dried in a dark place. 
The light intensity which produces in one second on such “ normal 
paper ” a shade equal to the “normal shade” is the Bunsen-Roscoe 
unit of measure of the chemical light intensity. Thus, if the color 
of the “normal shade” is sonchel. Tg ag Ie Peg a a n 
seconds, the corresponding light teenies are 1/1, 1/2, 1/3, 
a ee 
WIESNER’S INSOLATOR. 
The measurement of the chemical light intensity is carried on by 
means of a very simple device called an insolator (see fig. 6) and a 
stop watch. The insolator devised by Wiesner consists of a piece of 
soft wood 38 to 4 inches long, 24 to 3 inches wide, and about one-fourth 
of an inch thick, covered, with the exception of a narrow slit at one 
end (S), with black opaque paper. The strip of “normal paper ” 
(N. P.) is pushed in beneath the black paper, and the standard 
(“normal shade”) is placed in the slit beside the “ normal paper.” 
When a reading is to be made the insolator is held in a horizontal 
position. The photographic “normal paper” is drawn out until a 
section of it is brought into the open slit, thus exposing it to light. 
At the same moment that the “ normal paper ” is exposed to the light 
the time is taken. As soon as the “normal paper ” assumes a shade 
identical with the “ normal shade ” the watch is stopped and the time 
of exposure is recorded. The light intensity is determined by divid- 
ing the light value of the standard by the time which has elapsed 
from the beginning to the end of the exposure. Thus, if at a given 
light intensity eight seconds are required to produce the “ normal 
shade” on the “normal paper,” then the intensity 7=1:8=0.125, 
Bunsen-Roscoe units. 
As long as low light intensities are measured, the “ normal shade ” 
is Sufficient ; as soon, however, as high light intensities are to be meas- 
ured, in which the “ normal shade ” is assumed in fractions of a sec- 
ond, and therefore the time of exposure can not be accurately re- 
corded, standards of different light values greater than that of the 
“normal shade” become necessary. Such standards are readily ob- 
tained simply by a comparison with the “ normal shade.” Thus, if it 
is desired to obtain a standard 10 times greater than the “ normal 
shade,” a strip of “normal paper” is exposed in the insolator 10 
times the number of seconds required to reach the tint of the “ normal 
shade.” Since such standards to be of any value must remain un- 
