2 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 sKdes 

 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. i6 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 puUing out the draw-tube in- 

 creases the magnification. 



At least two sets of lenses accompany a compound 



