LIGHT OF DIFFERENT WAVE-LENGTHS BY FISH 



11 



a pivot. This strip is folded to form a short trough at each end and has a drop of 

 solder in one end to give weight enough to cause that end to fall when not held in 

 place. At the corners g and h of the frame (figs. 1 and 3) are placed electro-magnets 

 connected with a dry cell. When the switch outside the curtain is closed and the 

 free end of the food-bar is placed against the magnet, the bar is held in position 

 along the side of the frame, gh, above the aquarium. When the switch is opened, 

 the weighted end of the food-bar falls so that it is beneath the water surface imme- 

 diately above the front edge of the positive stimulus plate, at which point it is stopped 

 by a peg in the frame efg/i. Food placed in the shallow trough is thus allowed to 

 fall immediately in front of the stimulus patch. When the stimulus patch is shifted 

 from side to side, the food-bar is removed from the pivot and the ends reversed. 





r 



5!! 



L. 



^XJOfer/rrren/ A^i/ar/u/r?. 





L« ^6 cm. 



Fig. 4 



Section of aquarium and lamphouse along line xy in figure 1. 



A'^, Nemst glower in cross section. 



0, level of slit. 



RF, ray filter. 



SL, cylindrical lens in kit ki. 



S, stimulus plate of ground opal glass. 



