LIGHT OF DIFFERENT WAVE-LENGTHS BY FISH 7 



(See section H.) Such differential instinctive responses 

 may show the ability of the fish to discriminate, but they 

 persist only a short time. To secure responses persisting 

 for a longer period the fish were fed before one of the 

 stimulus patches, and thus a food association with the one 

 patch of light was built up in order to test the ability of 

 fish to discriminate between the two lighted areas. 



To secure patches of light of definite form on the stimu- 

 lus plates two cylindrical lenses {SL in fig. 2) are mounted 

 in a frame (indicated by EG in fig. 2) which rests above the 

 sub-compartments /' and 1". The major axis of each 

 lens is across its sub-compartment. These lenses par- 

 allelize the rays in one plane and give two stimulus patches 

 of equal size, having sharp parallel upper and lower edges 

 across the opal glass plates. The lower margin of each 

 patch is always adjusted so as to coincide with the lower 

 margin of its plate. Control of wave-length of light is 

 obtained by colored filters ( RF in fig. 2) laid above the 

 cylindrical lenses. A blue filter of gelatine and a red one 

 of ruby glass are provided and may be shifted from side 

 to side, so that patches of light of either color may be used 

 on either plate. The light for the illumination of the stimu- 

 lus plates comes from two single-glower Nernst lamps {N 

 in fig. 2) contained in a lamphouse. The house rests upon 

 the aquarium, above compartments 1' and 1", and its cross 

 section is nearly that of these sub-compartments taken 

 together. It is divided by a vertical partition {M'C in 

 fig. 2), which is an upward extension of partition C of the 

 aquarium. Access to its interior is through a door above 

 the end of the aquarium, which is not visible in figure 2. 

 The long lamphouse extends nearly to the ceiling, from 

 which it is so suspended that it may be swung out free 

 from the aquarium. The lamps remain always at the 

 same height in the lamphouse, and the intensity of the 

 light reaching the plates is controlled by a slit (O in fig. 2) 

 of variable width placed beneath each lamp at right angles 

 to its glower. Thus by varying the width of the slits and 

 by shifting the filters a patch of either white or colored 

 light of any desired intensity may be secured on either 

 plate. 



