86 NATURALISTS’ ASSISTANT. 
application : by sliding tube, and by rack and pinion. — For 
the novice 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 pimion 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”’ 
(7. e., 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 cf two or three lenses 
mounted in a short tube (Huygenian oculars), or the lenses 
