tube at the upper end, is moved up and down in an outer 

 sheath, which is fastened to the arm. The milled ring 

 is grasped by thumb and fore and middle fingers and 

 pushed down and drawn up with a spiral motion. It 

 is not to be commended except for economical reasons, 

 as it lacks firmness, wears out quickly from the con- 

 siderable friction, endangers the object and the 

 objective from the liability to sudden or jerking 

 motions and does not well permit the application of 

 the double nose-piece. While a clamping ring which 

 fastens the tube in a fixed position is provided in some 

 instruments, 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 and pinion adjustment is by far preferable 

 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 adjustment, 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 with straight cut teeth. In order 

 to make the pinion operative, bearings are provided for 



