Watson, Color Fision in Monkeys. 7 



openings, J-J^, and J 2'-^ i- The Avidtli of these openings is controlled 

 by means of the micrometer screw system Col, ES, K, B, the mechan- 

 ism of which is well known (i. e., turning Cal, e. g., forces the nnt, 

 K, backwards or forwards as the case may be and consequently the 

 edge of the jaw at J advances or recedes from J^). 



The two small jaws, J^ and J 2, must be moved by hand. They are 

 held in place (i. e., in the grooved track of the slit) by means of 

 a small bowed spring. Since they are cut accurately to fit the 

 track in which / and J3 slide, they are held firmly in vertical posi- 

 tion by means of the spring. If it is desired to have the opening, 

 J- J I, admit some other part of the spectrum, the apparatus easily 

 permits it. Suppose we desire to have the opening at 6 instead of 

 at 3.65, as it stands in the diagram (see relation of scale SC to 

 index /). We turn the screw, Cal, until / falls at 6. The small 

 jaw, Ji, is then pushed up flush against J. J is then pushed 

 forward, carrying Jj with it for whatever wndth of slit is desired ; 

 / is then backed again to 6. This leaves the opening, J-Ji, in its 

 new position optically perfect. By means of the micrometer screw 

 head this distance is made accurate to 1/1000 mm. This would 

 leave a much wider space at than before. This variable opening 

 at is closed with a strip of black cardboard. Four tiny points, 

 p, Pj. P2, P:^, on the jaws, J^ and J 2, facilitate this.'^ 



The position of this slit in the system can be inferred from an 

 examination of Fig. 2 : L.,, of Fig. 1 is sho^^^l on the right in Fig. 

 2 ; S2 is at the focus of this lens, 24" distant. The two small 

 vertical mirrors, il/^ and .1/;., placed at an angle of 4.5° so as to form 

 a horizontal V (with apex directed towards ^'2 and midway between 

 the two openings) serve to catch the two selected beams (e. g., red 

 and green) issuing from the openings, J-Ji and J-yT:^ of S-. and to 

 reflect them to the mirrors, ilf^ and M- resjiectively. These latter 

 two mirrors in turn reflect the two beams in a parallel way doA\Ti 

 the room to the screen. The width between these two mirrors 

 can be adjusted to any desired distance. Since the rays as they 

 issue from the openings of /S'o are diverging, we have a broad, diffuse, 



'The slots, SI and »S7,. serve to admit bolts for attaching slit as a whole 

 permanently in its vertical position. 



