306 Tue Microscope. 
The lower end bears a diaphragm whose aperture sometimes 
contains the Society screw, so that very low power objectives 
may be placed there and used while entirely within the body. 
This is often a great convenience in the employment of such 
objectives as the five, four, or three inch, which have such an 
exceedingly long working distance that occasionally the body 
must be raised so high above the stage that it will run off the 
rack before the focus is obtained, but if the objective is on the 
diaphragm of the draw tube it may be approximately focussed 
by pulling out that tube, the focussing being completed by the 
rack and pinion on the body. These low power objectives are 
at times very useful in the study of large objects, where not much 
amplification is desired. 
What are styled Students’ stands often have the draw tube 
diaphragm supplied with the Society screw. It is not restricted 
to first class instruments, as it is almost a necessity on any stand, 
for it not only may carry the low powers, but also the Amplifier, 
and the analyser of the Polariscope. As a diaphragm of some 
kind must always be in the tube, and as the Society screw can 
add but the veriest trifle to the cost of the stand, the purchaser 
might do well to seek an instrument with this convenience. 
The draw tube is also often externally graduated to parts of 
an inch or of centimeters or both, so that the distance to which 
it is extended may be recorded and a desirable result be repro- 
duced at any future time. The only graduation on the cheaper 
stands is a single circle engraved at the point to which the tube 
must be extended to make the body of the standard length. 
When the part is to be drawn out, do so with a strong and 
steady pull, holding the body firmly in position at the same time, 
otherwise it may be run off the rack and entirely off the arm. 
Do not turn the draw tube from side to side while extending it. 
The latter method puts a severe strain on the rack and pinion, 
and on the fine adjustment if it is at the back of the arm. 
When it is to be pushed in, grasp the milled head of the pinion, 
or hold the body so that the latter can not move, while the tube 
is slowly and steadily pressed down. If these precautions are 
not taken, and the objective is on the body, it may be forced 
against the slide, or the air compressed within the body may 
throw out the eye-piece. 
