200 CHARLES HARLAN ABBOTT 



At one end of the table was placed a wooden box, black both 

 inside and outside, containing the source of light — a 60-watt, 

 250- volt Mazda lamp. To insure symmetrical illmnination of 

 both sides of the field, the lamp was placed in a vertical position 

 in the box. The light passed out through a horizontal slit, 

 41 X 8 mm., in a diaphragm. The lower edge of this slit was 5 

 mm. above the table top, insuring the illumination of the surface 

 of the table and at the same time allowing the light to strike- 

 the eyes of the isopod from a horizontal direction. The slit was 

 covered with thin paraffined paper in order to make a source of 

 even illmnination. The relation between the table top and the 

 source of light is shown in figures 1 and 2. 



w 



Fig. 1 Vertical diagram of apparatus: mn, table top; D, diaphragm; s, slit 

 in diaphragm through which light passes; L, light; W, rectangular jar of distilled 

 water to cut out heat; jp, position where animals were placed; m-p, path of light 

 to point p. 



A horizontal field, 60 x 40 cm . divided into squares 10 cm. on 

 a side, was marked out o' \,.. part of the table next to the 

 light (fig. 2). This gave an area with the intensity gradually 

 diminishing both from the source of light to the opposite end 

 of the field and from the center to the sides. As the slit in the 

 diaphragm through which the light passed was only 41 mm. in 

 length, dark corners (fig. 2, c and c'), were left at the sides and 

 not included in the experimental field. Black strips of wood 

 were placed along the lines ef and jk to shut off possible side 

 reflections. The diaphragm was 5 mm. from the edge of the 

 field. 



In most of the experiments the animals were exposed to the 

 light after they had been placed in the center of the field, i.e.. 



