28o A. M. Banta 



tolightissufficient to account both ior the occurrence of Caecidotea 

 in caves and subterranean waters in general and foi; the virtual 

 non-occurrence of Asellus in such situations; for the negative res- 

 ponse of Caecidotea to hght would aid in directing it into caves 

 and keeping it there; but Asellus after being in darkness becomes 

 positive, and therefore would move toward the light, i.e., out of a 

 cave, in case it had by chance made its way into one. 



II. VERTICAL ILLUMINATiON 

 /. Methods and Apparatus 



The apparatus used in the experiments with vertical illumina- 

 tion is shown in vertical section in Fig. 6 (p. 281), a dark and a 

 light field were secured by using the same two tanks as in horizon- 

 tal illumination and causing one-half of the inner tank to be illum- 

 inated by vertical rays while the other half was kept as dark as 

 possible. A rectangular lamp-container (LC), similar to the one 

 used in the experiments with horizontal illumination, was sus- 

 pended in a vertical position by means of two stout cords passed 

 through pulleys so that it could be raised or lowered as desired. 

 A broad V-shaped piece of blackened sheet iron (C) was placed as 

 a roof over the top of the lamp container in such a way as to pre- 

 vent the light from escaping above into the room and at the same 

 time to afford a ready means for the escape of the heated air from 

 around the lamp. Within the lamp-container at one side was fitted 

 an adjustable partition {P) made of two thicknesses of paste- 

 board. This partition was lengthened at the lower end by means 

 of a heavy card-board fastened between the two thicknesses of 

 pasteboard in such a way that it could be moved freely in the 

 plane of the partition. The lower edge of this partition was made 

 to bisect the inner tank, fitting into it closely and extending down 

 to the surface of the water when the tank was filled to a depth of 

 3 cm. The partition was practically light proof; but in order the 

 more effectually to shut out all light from the dark end of the tank, 

 that end was carefully covered over with a black cloth supported 

 by a large piece of blackened pasteboard {S). Black cloth was 



