Reactions of Isopods to Light 30 1 



ally than Caecidotea, smaller percents of Asellus than of Caecidotea 

 entering and remaining within the light region. Asellus very 

 generally reacts at once to the sudden change in illumination at 

 the plane of separation between the illuminated and dark regions, 

 whereas Caecidotea only occasionally reacts at that plane. 



III. ILLUMINATION BY DIRECT SUNLIGHT WITH THE RAYS AT 

 RIGHT ANGLES TO THE LONG AXIS OF THE TANK 



As in the experiments with vertical illumination, the tank was so 

 arranged in this series as to have a dark and an illuminated region 

 with a sharp plane of separation between the two. This series of 

 experiments was made at a south window in the basement of the 

 Museum. The available space directly within this window was 

 large enough to enable one to make use of the direct sunlight for only 

 a limited period (about two hours daily), but fortunately this period 

 came at about mid-day. Just what the sun's luminosity ordin- 

 arily is at mid-day in Cambridge was not determined. Naturally 

 so many factors affecting the luminosity are concerned that any 

 determination or estimate of it at one time would have little value 

 for any other time. To my eye the brightness at the tank under 

 direct mid-day sunlight ordinarily seemed approximately twice 

 as great as that produced by the 6983 CM. artificial light so often 

 used. In these experiments the inner glass tank filled with water 

 to a depth of 3 cm., was used without the outer tank. Under 

 one half of the tank was placed black cloth of several thicknesses. 

 This extended exactly to the middle of the tank, and was fitted to 

 the end and sides and over the top of that half. To further ex- 

 clude the light from this part of the tank, a black card-board par- 

 tition extended down from the edge of the cloth at the median 

 plane. This partition just cleared the surface of the water, so that 

 the only light which entered the dark end came through the water 

 below the partition. Of course the so-called dark end was consid- 

 erably illuminated owing to the diffusion of hght through the water 

 but the plane between the illuminated and dark regions was never- 

 theless very sharp, so that the contrast between the two wasstrong. 

 The tank was placed upon a light but rigid box so that it could 



