622 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
it is geared into the rack. The teeth of the rack should be 18 to the 
inch and cut at an angle of about 15° less than a right angle. When 
the pinion has been turned with its proper bearings and the milled head: 
Fig. 62. Fig. 63. Fig. 64. 
fitted on, the borings should be brooched taper ; the hollow and square 
hole in fig. 60 a should be made to receive it and the plate (fig. 60 b) fixed 
on with two screws. The bar on optical tube (fig. 61 b ) must be care- 
Fig. 65. 
Fig. 66. 
fully adjusted to female fitting (fig. 61 a), and a little pumice powder and 
oil can be used to secure a perfect fitting. 
The tube beneath the stage for carrying the apparatus should be of 
