60 ROBERT M. YERKES AND JOHN B. WATSON 



of the cabinet for a distance of 25 cm. Probably the chief 

 advantage of the cabinet comes from the fact that with it ex- 

 periments may be carried out in full daylight illiimination. 



3. Description of color-spacing and reversing device 

 The various parts by means of which the two beams are 

 selected, spaced, reversed and finally projected upon the plaster 

 of paris surface, are all assembled upon the iron casting Y, 

 fig. 9. This consists of a base Y' thick and 3" wide, mounted 

 upon three levelling screws, two of which are seen in i and 2, 

 and two uprights, 3 and 4. The upright pieces are notched on 

 the back so as to support the metal track 5. This track con- 

 sists of two ^" square iron bars, 60 cm. long, bolted together 

 parallel by straps, 6 and 7. The track is held to the uprights 

 by means of clamps 8 and 9. It will be seen that the track 

 can be clamped to the casting in any desired position to the 

 right or left. In the central point in the length of this track, 

 there is attached another short track, 10, normal to it. The 

 front upper part of each of the uprights 3 and 4 is notched so 

 as to receive the slit-wall 11, containing the double slit (see 

 fig. 10). The front plate of this slit-wall is fastened to a rect- 

 angular frame, 12, made of \" square brass rods. To the upper 

 face of the low^er horizontal bar of this frame is attached a 

 grooved track, 13, the beveled sides of which are fastened to 

 a horizontal plate by means of screws, two of which appear 

 at 14 and 15. 



Running in this grooved track are to be found two small 

 prism tables, 16 and 17, which can be moved by hand and 

 clamped in any desired position by arrangements not shown in 

 the cut. To these tables are attached two total reflection prisms 

 with 9 mm. face, 18 and 19 (m^ and w, of fig. 6). The use to 

 which these prisms are put is mentioned on page 47. The plate 

 and frame holding the double slit mechanism is shown in front 

 view in fig. 10. 



In the present drawing is seen the back view of this double 

 slit mechanism. The letters O, J, Ji, J,, Ju, have the same 

 significance as in fig. 10. In the back of the slit-wall is the 

 window W, the slit jaws, J, Ji, J 2, J 3, slide in front of this. Jaws 

 Ji and J 2 are held firmly against the track by means of spring 

 clips, c and d. The two slit openings described in fig. 10 are 



