86 NATURALISTS' ASSISTANT. 



application : by sliding tube, and by rack and pinion. For 

 the novi:e the latter is the better method but in the hands 

 of the experienced person equally good results are readily 

 obtained by the former plan. With the sliding tube, the tube 

 carrying the lenses is made to slide in a closely fitting collar 

 by a screwing motion. The rack and pinion dispense with 

 the collar and move the tube by a toothed wheel, working 

 into a straight bar fitted with similar teeth. The great ob- 

 jection to this is that the teeth wear rapidly, thus allowing 

 more or less "play" and causing the tube to move by jerks, 

 a serious disadvantage. 



The methods employed for securing the fine adjustment 

 are still more numerous. Some move the "nose piece" 

 (/'. <?.,. move the objective without altering the position of the 

 tube) ; others move the tube, and still others move the stage. 

 These last forms are the worst of all and should never be em- 

 ployed. Between the other two and the various methods 

 employed for each, there is but little choice when well made. 

 The purchaser should, however, -always see that the fine ad- 

 justment works easily, responds to the slightest turn of the 

 adjusting screw, is durable, and can be regulated for very 

 short distances. If proper precautions are taken by the maker 

 to secure an absolutely straight motion without any lateral de- 

 viation cr shake, it is perhaps best to have the whole tube 

 move by the fine adjustment, rather than the objective alone. 



The lenses are the important portion of the instrument 

 and upon their perfection its value almost entirely depends. 

 The eye-piece may either consist of two or three lenses 

 mounted in a short tube (Huygenian oculars) , or the lenses 



