iS MICROSCOPE AND ACCESSORIES. 



focal it will be necessary to lower the tube somewhat to get the image 

 in focus.* 



Pull out the draw-tube (Fig. 10) 4-6 cm., thus lengthening the body 

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

 the microscope somewhat. 



§49. 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. 



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

 as before, elevate the body of the microscope and remove the ^ in. 

 (18 mm.) objective as indicated. Put the \ in., (5 mm.) or a higher 

 objective in place, and use a low ocular. 



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

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

 fore (§ 48), and lower the body 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 body very slowly 

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

 adjustment (Fig. 10), first one way and then the other, if necessary, 

 until the image is sharply defined. 



Note that this high objective must be brought nearer the object than 

 the low one, and that by changing to a higher ocular, if the oculars 

 are not par-focal, or lengthening the body it will be found necessary 

 to bring the objective still nearer the object, as with low objective 



(§ 48). ^ 



§ 51. Always Focus Up, as directed above. If one lowers the body 

 only when looking at the end of the objective as directed above, there 

 will be no danger of bringing the objective in contact with the object, 

 as may be done if one looks into the microscope and focuses down. 



When the instrument is well focused, move the object around in order 

 to bring different parts into the field of view (§ 33). It may be neces- 

 sary to re-focus with the fine adjustment every time a different part is 

 brought into the field. In practical work, one hand is kept on the fine 

 adjustment constantly, and the focus is continually varied. 



* 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. 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. 



