METHODS OF STUDYING VISION IN ANLMALS 53 



8" focus, 3" aperture, supplied with rack and pinion). The 

 parallel rays then strike the face of a dense flint glass prism, 

 25, (face 4"x4")- The excess white light reflected from the 

 face is thrown into the small dark box, 26. 



The refracted beam passes through the objective, 28, (Zeiss 

 "extra rapid" focus portrait lens, 3" aperture, 20" focus). This 

 lens is not regularly supplied with rack and pinion, and is there- 

 fore mounted on a carriage, 29. This objective brings the 

 beam to a focus on the silvered face of a vertically placed slit 

 wall or table, 30. The mechanism of this is taken up separately 

 in figs. 9 and 10. The wall carries four sliding jaws, which fonn 

 two slits whose widths are adjusted by means of the micrometer 

 screw-head, 31, and by one not shown in the cut. The casting, 

 Y, which holds the slit-wall, also carries a metal track, 35. A 

 metal support (see 22 of fig. 9) which may be fixed to the metal 

 track 35 at any desired point, bears a sliding mechanism, 36, 

 (N of fig. 6) for reversing the beams. The place of this reversing 

 mechanism on track 35 varies slightly, depending on the mono- 

 chromatic beams selected, but is shown in the cut as occupying 

 the middle of the track. On each side of this is mounted a 

 small projection lens, 32 and t,t,; near the ends of the track are 

 mounted the prisms 37 and 38. A third projection lens, 34, is 

 mounted upon a metal track projecting at right angles to the 

 main track. (These numbers refer to the same objects as in 

 fig. 6.) Two small prisms lying between lenses 32 and t,t, are 

 not clearly shown in this cut, but appear separately in fig. 9. 

 The whole system mounted on Y, which selects, spaces and 

 reverses the beams, will be made clear by reference to fig. 9. 



Three small windows, G, R and Gj, 2" x 2" each, are made 

 in the wall D, 25 cm. between centers. Only two of these 

 windows are in use at any one position of the colors. 



In room /, a stimulus carriage, H, is mounted upon the trestle- 

 work F, so as to travel right and left. The rectangular wooden 

 frame 64 carries the tw^o 3" x 5" speculum mirrors, L and M, 

 incHned at an angle of 45 degrees to the two incident beams. 

 These mirrors are attached to the framework at a height of 14'' 

 from the floor, W, of the stimulus carriage. The two mirrors 

 reflect the beams vertically downw^ard to the rectangular plaster 

 of paris surfaces, S and Sj. F and Fj show the two food boxes. 

 A partition, 65, separates the two food compartments. In 



