METHODS OF DETERMINING TOLERANCE. 35 
measuring light in the forest so far have been based to some extent 
on the measurement of its luminous intensity, but principally on the 
measurement of its chemical intensity. 
MEASUREMENT OF LUMINOUS LIGHT INTENSITY. 
In physics the usual method of determining the intensity of a 
luminous source is to compare it with a light of known intensity. 
For this purpose there have been adopted a number of hghts which 
are used as the standard of comparison; among these may be men- 
tioned the “standard candle,” the Hefner-Alteneck lamp, and the 
Vernon Harcourt lamp. The use of such standards in field work, 
however, has been found very inconvenient and impracticable, and 
a number of attempts have been made to devise an instrument by 
which the luminous intensity of light could be measured without 
the use of the standard light. None of these attempts has, however, 
resulted in a generally adopted instrument. The best-known instru- 
ments for this purpose are the smoked-glass photometer and the 
polarization photometer, for which the inventor Wagner (1907), 
claims great simplicity and accuracy. Nobody, however, except the 
inventor himself has used these instruments for field work, and no 
definite results of any permanent value have been obtained with 
them. 
The principle upon which these two instruments are based is that 
of measuring the amount of hght which is intercepted, in one case 
by a graduated, slding, smoked glass of uneven thickness (wedge 
shaped), and in the other case by two Nichol’s prisms whose planes 
of vibration form various angles with each other. If the amount of 
light intercepted by the thinner and thicker portions of the wedge- 
shaped smoked glass, or by the two prisms at various angles of their 
planes of vibration, is known, then the intensity of light in the open 
and in the forest may be determined. Thus, if the observer looks 
through the tube of the smoked glass photometer in the open and 
obtains complete absorption of light by the smoked glass at a point 
at which its light absorption is equal to 25 units, while in the forest 
complete darkness is obtained at a point at which its absorption of 
light is equal to 20 units, then the light intensity in the forest is 
to that in the open as 20 is to 25, or, expressed in per cent, the light 
in the forest 1s 80 per cent of that in the open. 
In the case of the polarization photometer, the amount of light 
_ absorption in the open and in the forest is measured not by a smoked 
glass of uneven thickness but by the angle between the planes of 
vibration of the two Nichol’s prisms. 
These two photometers are very simple, but they are not free from 
error, since with both the accuracy of the measurements depends 
