CH. II.] LIGHTING AND FOCUSING. 37 



Hold the head at about the level of the stage, look toward the win- 

 dow, and between the object and the front of the objective ; with the 

 coarse adjustment lower the tube until the objective is within about 

 half a cm. of the object. Then look into the microscope and slowly 

 elevate the tube with the coarse adjustment. The image will appear 

 dimly at first, but will become very distinct by turning the tube still 

 higher. If the tube is raised too high the image will become indistinct, 

 and finally disappear. It will again appear if the tube is lowered the 

 proper distance. 



When the microscope is well focused try both the concave and the 

 plane mirrors, in various positions and note the effect. Put a high ocu- 

 lar in place of the low one (§ 40). If the oculars are not par-focal it 

 will be necessary to lower the tube somewhat to get the image in focus.* 



Pull out the draw-tube 4-6 cm., thus lengthening the body of the 

 microscope, and it will be found necessary to lower the tube of the mi- 

 croscope somewhat. (For reason, see Fig. 57). 



§ 69. Pushing in the Draw-Tube. — To push in the draw-tube, 

 grasp the large milled ring of the ocular with one hand, and the milled 

 head of the coarse adjustment with the other, and gradually push the 

 draw-tube into the tube. If this were done without these precautions 

 the objective might be forced against the object and the ocular thrown 

 out by the compressed air. 



§ 70. Focusing with High Objectives. — Employ the same object 

 as before, elevate the tube of the microscope and remove the 16 mm. 

 (^i in.) objective as indicated. Put the 3 mm. {}i in.) or a higher ob- 

 jective in place, and use a low ocular. 



Light well, and employ the proper opening in the diaphragm, etc. 

 (§ 64). Look between the front of the objective and the object as be- 

 fore (§ 68), and lower the tube with the coarse adjustment till the ob- 

 jective almost touches the cover-glass over the object. Look into the 

 microscope, and with the coarse adjustment, raise the tube very slowly 

 until the image begins to appear, then turn the milled head of the fine 

 adjustment (frontispiece), first one way and then the other, if neces- 

 sary, until the image is sharply defined. 



In practice it is found of great advantage to move the preparation 

 slightly while focusing. This enables one to determine the approach 



* Par-focal oculars are so constructed, or so mounted, that those of different pow- 

 ers may be interchanged without the microscopic image becoming wholly out of 

 focus (Fig. 31, note, p. 23). When high objectives are used, while the image may 

 be seen after changing oculars, the instrument nearly always needs slight focusing. 

 With low powers this may not be necessary. 



