ADJUSTMENT TO LIGHT 75 



made only on clear clays, except where the light values of cloudy 

 days are desired for special purposes. After a strip has been 

 completely exposed, it is removed in the dark, and a new one 

 is put in place. The former is carefully labeled, and dated upon 

 the back, and is then filed with the same care as the standard. 



92. Comparison with the standard. The light intensity de- 

 noted by each exposure is ascertained by comparing the latter 

 with the standard. The strips are placed alongside of each other 

 in gaslight of fair strength, and the exposure is moved along 

 the standard until the tint that matches it is secured. With a 

 little practice this may be readily done. Skill and certainty in 

 making the tints match are obtained by comparing the exjDosed 

 strip with the standard a second time. If this is done without 

 reference to the first results, the two comparisons serve as a valu- 

 able check upon each other. When the proper match is secured 

 for a particular exposure, the comparative light intensity is found 

 by dividing the length of exposure in seconds by the length for 

 the standard tint. Thus, if an exposure in deep shade for ISO 

 seconds matches the 2-second standard, the light is 90 times 

 more diffuse or weaker than the sunlight. The latter is taken as 

 unity, and the light intensity of the shade is written in the form .01. 



93. Causes of variation in light intensity. The primary object 

 of light readings is to determine the amount of light in various 

 habitats, as a basis for explaining the differences shown by the 

 plants in them. The real differences between habitats arise from 

 the presence or absence of a primary layer, as well as from the 

 character of the latter. In striving to measure such differences, 

 it is absolutely necessary to avoid errors arising from the con- 

 dition of the sky, or from time or place. Clouds have a marked 

 effect upon the amount of light, almost invariably by reducing 

 it. This difficulty, as already indicated, is entirely eliminated 

 by taking readings upon clear days alone, avoiding even those 

 when the sky is slightly hazy. 



94. The effect of time. The intensity of the light varies 

 throughout the day and the year. The daily maximum occurs 

 at noon ''sun-time," a point which itself moves back and forth 

 through the year. The annual maximum falls on June 22, the 

 minimum upon December 22. The daily minimum is reached 

 at nightfall, and it lasts until dawn. In both cases the greatest 

 light intensity occurs when the sun is at its highest altitude, i.e., 



