LIGHT 55 
deep. In the shade where the action is slower, this difficulty is not felt. 
For this reason it is usually desirable to check the results by a multiple stand- 
ard, and in the case of selagraph records, where the various exposures show 
a wide range of tint, light values are obtainable only by direct comparison 
with the multiple standard. The exact matching of exposure and standard 
requires great accuracy, but with a little practice this may be done with 
slight chance of error by merely moving the exposure along the various 
tints of the standard until the proper shade is found. The requisite skill is 
soon acquired by running over a strip of exposures several times until the 
comparisons always yield the same results for each. The margin of error 
is practically negligible when the same person makes all the comparisons, 
and in the case of two or three working on the same reading the results 
diverge little or not at all. The efficient difference for light is much more 
of a variable than is the case with water-content. It has been determined 
so far only for a few species, all of which seem to indicate that appreciable 
modification in the form or structure of a leaf does not occur until the 
reduction in intensity reaches .1 of the meridian sunlight at the June sol- 
stice. The error of compariscn is far less than this, and consequently may 
be ignored, even in the most painstaking inquiry. 
Readings 
85. Time. The intensity of the light incident upon a habitat varies peri- 
odically with the hour and the day, and changes in accord with the changing 
conditions of the sky. The light variations on cloudy days can only be de- 
termined by the photometer. While these can not be ignored, proper com- 
parisons can be instituted only between the readings taken on normal days 
of sunshine. The sunlight varies with the altitude of the sun, i. e., the angle 
which its rays make with the surface at a given latitude. This angle reaches 
a daily maximum at meridian. The yearly maximum falls on June 22, and 
the angle decreases in both directions through the year to a minimum on 
December 22. At equal distances from either solstice, the angle is the same, 
e. g., on March 21 and September 23. At Lincoln (41° N. latitude) the 
extremes at meridian are 73° and 26°; at Minnehaha (39°) they are 75° 
and 28°. The extremes for any latitude may be found by subtracting 
its distance in degrees north of the two tropics from 90. Thus, the 5oth 
parallel is 26.5° north of the tropic of Cancer, and the maximum altitude of 
the sun at a place upon it is 63.5°. It is 73.5° north of the tropic of Capri- 
corn, and the minimum meridional altitude is 16.5°. 
The changes in the amount of light due to the altitude of the sun are pro- 
duced bv the earth’s atmosphere. The absorption of light rays is greatest 
near the horizon, where their pathway through the atmosphere is longest, 
