20 ELEMENTS OF APPLIED MICROSCOPY. 
ferent size in the cruder microscopes, and by an Iris 
diaphragm in more elaborate instruments. 
‘The arm is borne by the upper part of the pillar which 
moves in the lower part, and this motion, called the fine 
adjustment, is regulated by a milled head at the top. 
By this means the distance between the lenses and the 
object may be varied with great delicacy. More exten- 
sive changes in the position of the lenses are made possi- 
ble by the fact that the tube in which they are fixed slides 
up and down in the clamp which holds it at the end of 
the arm. This movement, called the coarse adjustment, 
may be regulated by hand or by a rack-and-pinion. 
The tube itself is divided into two portions, the tube 
proper and an inner cylinder, the draw-tube, which may 
be pulled out telescope fashion from its upper end. Into 
the lower end of the tube proper the various objectives 
may be screwed, while the eyepieces or oculars slip into 
the upper end of the draw-tube. When the draw-tube 
is pulled out the space between the two systems of lenses 
is increased, and in order that the real image produced 
by the objective may be visible through the eyepiece, it 
must be formed farther up than would ordinarily be the 
case. A reference to Fig. 16 will show that, according to 
the principles previously deduced, this can be accom- 
plished by bringing the object nearer the principal focus 
of the objective, and that the image of the object so 
brought nearer the principal focus will be correspond- 
ingly enlarged. Thus pulling out the draw-tube in- 
creases the magnification. 
At least two sets of lenses accompany a compound 
