63 



objective from the liability of sudden or jerking' 

 motions and does not well permit the application 

 of the double nose-piece. While the clamping- 

 ring is provided in some instruments, which will 

 fasten the tube in a fixed position, especially to 

 permit the use of double nose-piece, this again has 

 its disadvantages and is cumbersome. Therefore 

 it is strongly recommended not to purchase an 

 instrument of this kind if it can be avoided. 



The rack &&& pinion adjustment is by far prefer- 

 able in every respect and has stood the test of 

 many years, although efforts have been made to 

 introduce other methods, all of which, however, 

 have become obsolete. To be satisfactory and 

 lasting, it must be exceedingly well made and it is 

 safe to advise that any instrument with this adjust- 

 ment, which does not work well at the outset may 

 be regarded as a poor one. In late years the 

 pinion with spirally cut teeth and the rack with 

 diagonal ones has come into common use and is 

 better than the older form of straight cut teeth. 

 In order to make the pinion operative, bearings 

 are provided for it in the arm and its teeth engage 

 in the rack, which is fastened to the slide, which 

 also has its bearing in the recessed vertical length 

 of the arm, as shown in Fig. ^(>. 



This adjustment must meet the following con- 

 ditions and if it does not, the instrument may be 

 safely condemned as faulty. 



