62 ROBERT M. YERKES AND JOHN B. WATSON 



serves to clamp the post to the track. A gasket is placed between 

 nut and track. 



These posts are drilled with a |" hole, and are suppHed with 

 clamp-screws, 27, 28, 29, 30, 31 and 32. Posts 20 and 24 carry 

 prism tables, fixed directly to an upright |" in diameter. 35 

 is another plate of the same shape and size mounted above t,t, 

 by means of three screws (clearance holes are drilled for these) 

 two of which are seen at 36 and 37. The plate 35 is constantly 

 pressed upward against the screw-heads by means of spiral 

 springs, two of which are seen at 38 and 39, through which 

 the screws run to the tapped holes in 33. It will be seen that 

 this device enables one accurately to level plate 35, upon which 

 is fixed the total reflection prism 40. The table on the opposite 

 side, 34, is identical in structure. 



Each of the uprights, 21, 23 and 25, carries a lens holder, 42, 

 43 and 44 respectively. These holders carry the three small 

 achromatic projection lenses, 45, 46 and 47. 



Post 22 supports the reversing mechanism, which consists of 

 the fixed plate 48 mounted directly upon a |" rod. An upper 

 plate, 49, is hinged to this plate by means of pivot screws 50 

 and 51. The upper plate has a leveling device attached, similar 

 to that just described for plates 34 and 35. The upper plate 

 has two beveled strips, one of which is seen at 52, screwed to it 

 so as to form a track w^hich receives the sliding plate 53. The 

 two 19 mm. total reflection prisms 54 and 55 (m^ and ni^ of 

 fig. 6) used in reversing the beams are cemented to this sliding 

 track. The lateral movement of the slide is limited by two 

 stops, one of which, 56, is adjustable. The sliding plate itself, 

 53, is moved from side to side by means of the light gut cord, 

 57, attached to a metal bridge, 58. This bridge is broken away 

 in front for the sake of clearness. The opposite attachment 

 for cord 59 is not shown. It will be remembered that this cord 

 system is attached to block 46 (see p. 66). 



a. The double-slit mechanism and calibration. — Fig. 10 is a 

 horizontal drawing of the double-slit structure. It is con- 

 structed as follows: A heavy brass plate, A, 3-16" thick, 2f" 

 wide and 12" long, is attached by screws, 1-9 inclusive, to a 

 frame made of ^" square brass. The plate is slotted at SI and 

 Sli, so that it may be attached vertically by thumb-screws 

 to the planed surface of the uprights of Y (page 61). A window 



