LIGHT 



57 



approximately) ; between 8:00 a.m. and 4:00 p.m. the range in intensity is 

 from .90 to .98 merely. On December 22, the greatest intensity is .52, the 

 least .20 (the latter at 8 :oo a.m. and 4 :oo p.m. approximately). If the grow- 

 ing season be taken as beginning with the ist of March and closing the ist of 

 October, the greatest variation in light intensity at Lincoln within a period of 

 10 hours with the meridian at its center (cloudy days excepted) is from .33 to 

 .98. In a period of 8 hours, the extremes are .65 to .98, i. e., the greatest 

 variation, .3, is far within the efficient difference, which has been put at .9. 



ait 



Fig. 13. Chart for the determination of the sun's altitude, and the corresponding light intensity. 



For the growing period, then, readings made between 8 :oo a.m. and 4 :oo p.m. 

 on normal sunshiny days may be compared directly, without taking into ac- 

 count the compensation for the sun's altitude. Until the efficient difference 

 has been determined for a large number of species, however, it seems wise to 

 err on the safe side and to compensate for great differences in time of day 

 or year. In all doubtful cases, the intensity obtained by the astronomical 



