REACTIONS OF LAND ISOPODS TO LIGHT 



201 



30.5 cm. from the diaphragm and 20 cm. from the sides of the 

 field. A circle with a radius of 10 cm. was drawn about this 

 point as a center, and was divided into sixteen sectors, which 

 were numbered 1 to 8 on each side of the central axis m n 

 (fig. 2). 



The intensity of the light was measured by means of a Mac- 

 beth illuminometer, which was kindly loaned by Prof. W. E. 

 Kenerson, of the Engineering Department. Intensities in candle 

 meters (CM.) of the principal points of the field, determined 



Fig. 2 Horizontal diagram of apparatus: efghjk, experimental field, divided 

 into squares 10 cm. on a side, with a circle of 10 cm. radius at the center; mn, 

 central axis of field, receiving the most intense light; c and c', dark corners; B, 

 box containing light; L, light; D, diaphragm; s, slit in diaphragm; W, rectangu- 

 lar jar of distilled water to cut out heat. 



early in the experiments, are indicated in figure 3. The inten- 

 sities at three points along the axis 7n n, i.e., at 20.5, 30.5, and 

 40.5 cm. from the diaphragm, were determined with considerable 

 accuracy. The intensities of the other points indicated were less 

 easily determined, due to the angles at which it was necessary 

 to make the readings and to the rapidly decreasing light on the 

 sides of the field. This difficulty will account for the differences 

 in the readings from the two sides. The constancy of illumina- 

 tion throughout the experiments is indicated by a reading of 



