Single standards are exceedingly convenient in photometer readings, but they are open to one objection. In the sunshine it is necessary to make instant decision upon the accuracy of the match, or the exposure becomes too 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 standard, 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 solstice. The error of comparison 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 periodically 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 determined by the photometer. While these can not be ignored, proper comparisons 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 50th 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 Capricorn, and the minimum meridional altitude is 16.5°.