22 



THE MICROSCOPE 



is turned. The larger disks with the rack and pinion make up the coarse adjust- 

 ment, or coarse adjustment screws. By turning the disks gently, you can see 

 the effect on the tube. The coarse adjustment is used to change the distance 

 between the lenses and the object observed ; that is, to focus. Below the 

 coarse adjustment screws is a smaller pair of disks called the fine adjustment, or 

 fine adjustment screws. The fine adjustment is also used for focusing, but is 



much more delicate than the 

 coarse adjustment. 



III. The optical system 

 includes the part of the 

 microscope containing the 

 lenses. It consists of two 

 sets of lenses in metal cases. 

 The set placed at the lower 

 end of the tube, near the 

 ol^ject, is called the objective. 

 On most microscopes there is 

 a special attachment at the 

 base of the tube for carrying 

 two or more objectives con- 

 veniently ; this is the nose- 

 yiece. \Yith the use of the 

 nosepiece it is possible to change from one objective to another with no loss of 

 time. The objective that is in line with the tube is the one in use. In general, 

 the longer the objective, the greater is the magnifying power. Most micro- 

 scopes have low-power and high-poiver objectives. At the top of the tube, near 

 the eye, is a set of lenses in a metal case called the eyepiece. This is easily 

 taken out of the tube. 



IV. The illuminating system makes possible the adjustment and focusing 

 of the light on the object. It consists of the mirror, hung under the stage, and 

 of the diaphragm , inserted in the opening of the stage. The mirror usually has 

 two faces, one flat and one concave. The mirror can be turned in all directions 

 and is used for throwing a beam of light from the window (or suitable lamp) 

 upon the object and through the eyepiece, into the eye. The diaphragm 

 is an arrangement for enlarging or diminishing the amount of light coming 

 through the stage, by making the opening larger or smaller. 



V. Some microscopes have a joint in the pillar just below the stage, permit- 

 ting the upper part of the stand to be tilted into a more convenient position. 



The microscopes of to-day vary from the kind we use in our 

 school to those designed for elaborate research purposes. 



