THE MICROSCOPE AND CELL-STRUCTURE. 61 



coarse adjustment brings the outlines of the object dimly 

 into focus, but to get more accurate focussing (especially 

 when using a high-power objective) we use the fine adjust- 

 ment, by turning a screw at the top of the stand, behind the 

 body-tube. 



The object to be examined is placed on the stage, which 

 has two clips for fixing a slide in a definite position, but these 

 need not be used except for high powers, or while sketching. 

 There is usually a black plate, with holes of different sizes, 

 under the stage; this can be rotated so as to bring the 

 desired size of hole (diaphragm) under the central opening 

 of the stage. More expensive microscopes have an iris 

 diaphragm. A small hole is used with high power and a large 

 one with low power. 



For ordinary work two objectives are required, one for low 

 power (magnifying 60 to 80 diameters), and the other for 

 high power (300 to 400 diameters). The most useful are 

 1 inch or inch low power objective and ^ or ^ inch high 

 power objective. Of the two eyepieces the one with shorter 

 body and narrower eye-glass is the more powerful. 



Fig. 32. Dcrable Nosepiece. 



In most modern instruments the magnifying power can be 

 increased by having the body-tube constructed like a tele- 

 scope; the upper part (draw-tube), carrying the eyepiece, 

 can be drawn out. To avoid the inconvenience of having to 

 screw and unscrew a lens every time a change of magnifying 

 power is required, it is worth while to get a nosepiece 

 (Fig. 32), which is screwed to the lower end of the body- 

 tube and carries the two objectives ; by rotating it we can 

 quickly change from low to high power and vice versa. 



