54 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 
There is hardly any question that an instrument of this kind may 
be made to serve the present practical requirements of forest studies, 
for a measure of radiation intensity. It would be very desirable to 
have the necessary protection from air currents supplied, without 
intercepting some of the rays by a layer of glass. 
7. Photo-chemical photometers. — The objections which have been 
raised to the use of chemical reactions as a measure of the sunlight 
intensity can not be overcome. In addition to the fact that the other 
rays may not vary from time to time in the same proportion as the 
chemically active rays measured, it is somewhat questionable whether 
the result secured, as in the coloration of a photographic paper, is 
proportionate to the product of the light intensity and the time. 
In forest studies the photochemical method seems to serve one 
purpose fairly well, that being to obtain a measure of the density of 
the canopy. It is then assumed that the amount of light reaching 
the ground is to the total sunlight as the area of openings in the 
crowns is to the whole area ; or, in other words, that the light coming 
through these openings is unaltered in its passage. While technically 
there is also light below the crowns, which has been transmitted 
through the leaves, and the photographic paper may be sensitive to 
the rays in this class, it probably does not introduce any serious 
error, considering the purposes for which such measurements should 
be used. 
This crown-density determination should always be made by mov- 
ing the photometer through as great a space as possible during the 
few seconds of exposure. 
The Bunsen-Roscoe (55) unit of actinic light is the light required 
to produce on a standard paper a shade equivalent to that produced 
by the mixture of 1 part lampblack and 1,000 parts pure white zinc 
oxide. The details of the preparation of this " normal shade " and 
the " normal paper " are given by Zon and Graves (78) or may be 
obtained from the original citation given above. The object in men- 
tioning it here is simply to show that it is possible to carry on photo- 
metric observations on a fixed standard. 
Likewise, photographic-supply manufacturers have prepared a 
standard shade, and a standard paper for estimating light intensi- 
ties. One of the best known of the " exposure meters " is extremely 
simple in operation. In one opening of a small disk containing the 
standard paper is exposed the standard color, a permanent shade. In 
a corresponding opening may be seen a fresh area of the paper. It 
is only necessary to expose this to the light until it attains the stand- 
ard shade, noting the time required, to have a very close basis for 
computing the light intensity. It seems that this simple contrivance 
may serve the purpose of ecologists quite as well as more elaborate 
apparatus of the same type. 
