The Reactions of Plananaus to Light 



6s 



in the hood of the apparatus already described in Fig. i, the 

 minghng of their rays being prevented by the insertion of a ver- 

 tical diaphragm (Fig. 3, C), which extended from the region 

 between the lights down to the surface of the aquarium. In order 

 to place the diaphragm in position it was, of course, necessary to 

 remove the heat screen (Fig. i, C), the presence or absence of 

 which, however, would not have affected the results sought since 

 the water in the aquarium itself was nearly 2 cm. deep and thus 



Fig. 3 A, stationary light; B, sheet iron walls of hood; C, vertical diaphragm separating the two 

 lights; D, horizontal diaphragna; E, sheet iron roof of hood; F, plate glass aquarium floor; G, paraffine 

 wall of aquarium; HH, diaphragm to shut off reflections from wall of aquarium; /, wall of reflector box; 

 J, open side of box; X^, mirror; L, black tunnel; M, black draperies cutting off side light; A', table sup- 

 porting reflector box and end of tunnel; 0, movable light; P, track for movable light; ^, narrow, hori- 

 zontal diaphragm attached at right angles to the lower side of the diaphragm C, in order to prevent 

 the light rays from the two sources of light, A and 0,from overlapping. 



constituted an efficient heat screen. By keeping the hood sta- 

 tionary and causing one of the lights (Fig. 3, 0) to slide up and 

 down at will, it was possible to bring about various contrasts of 



