RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 55 
Weisner (76) used an instrument almost as simple as this, his 
standard shade and fresh paper being set in a groove in a block of 
wood, and appearing in openings of a layer of opaque paper. 
Clements (6) devised a photometer in which a narrow strip of 
" solio " or other sensitive paper may be held, having sufficient area 
for 25 exposures. This strip is placed on the periphery of one metal- 
lic ring, which fits snugly inside another. In the outer ring there is 
an opening one-fourth of an inch square, covered by a slide which 
is drawn back to make the exposure. In using this instrument the 
colors obtained on exposure are not directly compared with a stand- 
ard color. Bather, it is customary to make a scale of shades with 
each set of observations, consisting of, say, 10 exposures in full light, 
of 1, 2, 3, etc., seconds. In the later exposures, then, it is only neces- 
sary to keep within the limits of the "scale," and the time may be 
varied to secure the desired shade. If, for example, a 24-second ex- 
posure gives a shade corresponding to 7 seconds on the scale for full 
light, the relative value of the suppressed light is 7/24 or 29 per cent. 
The photochemical-photometer method is not satisfactory for any 
expression of the light in absolute terms, or for comparing quantities 
in one day or season with another day or season, or for comparing 
different localities. Of course, all exposures might be compared to 
some standard shade, but the operation is needlessly circuitous 
and is made the more difficult by the perishable nature of the record, 
the need for examining it in dim lamplight, etc. It is therefore be- 
lieved that, while this method has some value, a similar effort ex- 
pended in determining absolute light quantities will be much more 
profitable. 
In addition to the above there are instruments of the same prin- 
ciple by which more or less continuous records may be secured. In 
one such instrument, used by the Weather Bureau a number of years 
ago, the light entering through a very small opening made its im- 
pression as a band across the sheet of photographic paper on which it 
fell at successive moments. (See Clements (6), p. 51.) 
8. It is probable that several hundred kinds of comparison pho- 
tometers have been devised, all depending on an ocular comparison 
of the sunlight to be studied, with a light of known luminous powers. 
It is evident that such instruments deal only with the luminous rays, 
and while they rely upon the accuracy of the eye, the method cer- 
tainly has advantages over the photochemical method in dealing with 
that part of the spectrum which is least affected by changes in atmos- 
pheric absorption. It is not to be supposed, however, that the lumi- 
nous rays control plant activities. 
One of the simplest types of comparison photometers is the 
smoked-glass type used by Wagner and credited to him by Zon and 
Graves (78), although undoubtedly invented much before his time. 
