it in the arm and its teeth engage in the rack, which is 

 fastened to a slide and has its bearing in the recessed 

 vertical length of the arm, as shown in Fig. 31. 



Fig. 31. 



This adjustment must meet the following conditions 

 and if it does not, the instrument may be safely con- 

 demned as faulty : 



// must work with the utmost smoothness and with 

 not the least perceptible jar or grating. 



It must be free from lost motion when working with 

 the highest powers. 



The slide must be so perfectly fitted, that it shall 

 show no play when the tube is moderately forced from 

 one side to the other. 



It is opportune in this connection to state that all 

 fittings involved in the rack and pinion are necessarily 



47 



