64 LIGHT AND THE BEHAVIOR OF ORGANISMS 



which the beams of light fell and were absorbed. The 

 altitude of the triangular openings in the screen over the 

 lens was 7 mm. and the base 50 mm. The beams of light 

 thus produced were 14 mm. wide and 20 mm. high at the 

 focal point in the light grader, the place where the plumules 

 were exposed. At this point the light intensity in each 

 beam decreased from side to side at the rate of 2 ca. m.^ 

 per mm., it being 100 ca. m. at one side and zero at the 

 other. From these data the intensity at any part could 

 readily be calculated. In order to ascertain the intensity 

 to which the plumules w^ere exposed it was therefore neces- 

 sary only to learn their position in the field ; and to calculate 

 the difference of intensity on opposite sides it was sufficient 

 to know their diameter, the difference in all parts of the 

 field being 2 ca. m. per mm. width. 



During the first part of the work the movements of each 

 plumule were recorded by tracing its shadow cast upon a 

 sheet of paper held in a vertical position a few centimeters 

 back of it. The shadow was thus traced at the beginning 

 of the experiment and again at definite intervals. At first 

 only a few tracings were made in twenty-four hours. It 

 was however soon found that owing to marked circum- 

 nutating movements and to surprisingly indefinite lateral 

 deflections it was necessary to locate the position of the 

 plumules at 30 to 60 minute intervals (see Fig. 5). 



By this method only the lateral and vertical movements 

 of the plumule were recorded. There was no record of the 

 movement toward the source of light; in some of the later 

 experiments however this movement also w^as recorded. 

 A fine pointer was fastened so that the sharp end was 

 10 cm. above the tip of the plumule. A glass plate was 

 then fastened in a horizontal position one meter above the 

 pointer. By proper illumination the sharp end of the 

 pointer and the tip of the radicle could clearly be seen 

 throu^^h the glass plate, and it was not difficult to fix a 



^ The a bbreviation ca. m. will be used for the term candle meters through- 

 out this vc>lume. 



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