3 8 LIGHTING AND FOCUSING [CH. II 



§ 72. Focusing with Low Objectives. — Place a mounted fly's 

 wing under the microscope; put the 16 mm. (fi in.) objective in 

 position, and also the lowest ocular. Select the proper opening in the 

 diaphragm and light the object well with transmitted light (§ 64, 68). 



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

 window, 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 centimeter 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 raising 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 

 ocular in place of the low one (§43). If the oculars are not par- 

 focal it will be necessary to lower the tube somewhat to get the micro- 

 scope in focus.* 



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

 microscope ; it will be found necessary to lower the tube of the micro- 

 scope somewhat. (For reason, see Fig. 58.) 



§ 73. 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. 



§ 74. Focusing with High Objectives. — Employ the same 

 object as before, elevate the tube of the microscope and, if no revolving 

 nose-piece is present, remove the 16 mm. ( 2 3 in.) objective as indi- 

 cated. Put the 3 mm.(}s in.) or a higher objective in place, and use 

 a low ocular. 



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

 powers may be interchanged without the microscopic image becoming wholly out 

 of focus (Fig. 31). 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. 



Objectives are also now commonly mounted in the triple or double revolving 

 nose-pieces (Figs. 36, 36 a) so that if one of the objectives is in focus either of the 

 others will be approximately in focus when turned into position. This is a ver}' 

 great convenience. 



