LIGHT OF DIFFERENT WAVE-LENGTHS BY FISH 



13 



tion of one-half of the lamphouse and can be slid into a groove in either half of the 

 house. Shding in this frame is a pair of wide steel plates, each 48 mm. by 170 mm. 

 with beveled knife-edges. The wide plates can be brought close together when a 

 narrow slit is desired, or separated for slit apertures up to 4 mm. For greater slit 

 widths two other pairs of plates 35 mm. and 25 mm. by 170 mm. are provided to 

 replace the wider ones. The widths of the slit openings can be read on a millimeter 

 scale on the frame. For regulating the slit width of 0.1 mm. intervals, a series of 

 strips of accurately measured thickness are used. One of these strips is placed in 

 the slit, the plates are brought against it, and the strip is then carefully withdrawn. 

 The slit in position in the house is 3 cm. beneath the glower of either Nernst lamp. 

 The direction of the slit is at right angles to the length of the glower and to the 

 long axis of the lens. 



To prevent any variation in the appearance of the lower surfaces of the lenses 

 being seen by the fish, a curtain is hung across the top of the front of the sub-com- 

 partments 1' and 1", reaching to the level of the upper edge of the stimulus patches. 

 This is not shown in figure 2. 



e. Color-fillers. 



The color filters, each 9 cm. square, are placed as shown in figures 2, 4, 5, in a 

 photographic kit or frame 9 cm. by 9 cm., resting upon the lenses. The wave- 

 lengths of light transmitted by the blue filter are A4600 to A5245 with the maximum 

 in the blue; those transmitted by the red ruby glass are from A5890 to the end of 

 the visible spectrum. None of the yellow is let through by either filter, so that the 

 light used is from two separated parts of the spectrum. The blue filter is a Wratten 



Fig. 6 



Ground plan to show how the partitions are manipulated in testing the fish. 

 /'/", compartments for stimulus patches. 



A, movable partition with sliding door. 



B, T, movable partitions; T is used for cutting out single fish to be tested. 



A' B' , movable partitions swung out to allow fish separated from the others to 

 swim from 3 to 2. 



