LIGHT OF DIFFERENT WAVE-LENGTHS BY FISH 9 



the food-bar with its food. Next, by manipulation of the 

 movable partitions, the individual fish needed for an experi- 

 ment is allowed to swim from compartment 3 into compart- 

 ment 2. The further operations of lifting the sliding door 

 and releasing the food-bar when the fish has reacted suit- 

 ably are carried on from without the curtain, while obser- 

 vations are made through a peek hole. 



When experiments are not in progress, the ceiling light 

 is turned off, the yellow curtain drawn aside, the shades 

 at , the west window raised, and the movable partitions 

 taken from the aquarium. The fish then live in a black 

 pool into which the sunlight penetrates for a short time 

 each day. On the sides of the pool the fish may see the 

 yellow curtain and above it the gray of the ceiling. At 

 one end of the pool towers the black lamphouse. The 

 visual surroundings are then somewhat like the black 

 pool and the sky of the natural fish habitat. 



The tap water supplied to the aquarium is passed through 

 a Berkefeld filter. All fecal material and excess food are 

 daily removed from the aquarium. Frequently the aquar- 

 ium is almost emptied through a drain at the bottom; 

 then it is refilled. By these means the water is kept uni- 

 formly clear and its light transmission constant. 



That the fish may have all necessary food substances, 

 pieces of angle worm, fish, chicken, or bits of boiled egg 

 or fish food are occasionally substituted for the usual 

 scraped beef. The fish are healthy and active, and are 

 growing rapidly after over two years of experimentation. 



3. Detailed Description o? the Apparatus^ 



a. The Experiment Aquarium. 



The aquarium is 22 cm. deep, 66 cm. long, and 25.4 cm. wide, inside measure- 

 ments. Each of the three movable partitions (A, B, T in figs. 1, 2, 6) measures 23 

 cm. high by 25 cm. wide, and has attached to its u pper edge a small rod of metal 

 extending 3 cm. beyond the ends. By hanging the projecting ends of this rod over 

 the edges of the aquarium, a partition can be set at different places across the aquar- 

 ium. One of these partitions (A) has an opening 10 cm. wide, extending 13 cm. 

 from the bottom. This is provided with a sliding door {D in fig. 2) 13 by 23 cm. 

 Opposite to the door when it is in place are the stimulus plates (S,S in fig. 2). Each 

 plate extends entirely across its sub-compartment and reaches from the floor at the 

 projecting end of the partition C to the surface of the water at the back of the sub- 

 compartment. The tap water, after passing through a Berkefeld filter, flows over 



* The further descriptions in this section and in section 4 are included especially 

 for those who may wish to duplicate the apparatus and methods. 



