38 LIGHTING AND FOCUSING. \CH. II. 



to the focal point either from the shadow or the color, if the object is 

 colored. With high powers and scattered objects there might be no ob- 

 ject in the small field (see § 46, Fig. 37, for size of field). By moving 

 the preparation an object will be moved across the field and its shadow 

 gives one the hint that the objective is approaching the focal point. It 

 is sometimes desirable to focus on the edge of the cement ring or on the 

 little ring made by the marker (see Figs. 61-65 § 118). 



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 tube of the microscope it will be found 

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

 objective. (For reason see Fig. 57). 



§ 71. Always Focus Up, as directed above. If one lowers the tube 

 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 (§ 46). 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. 



§ 72. Determination of Working Distance. — As stated in § 57 

 this is the distance between the front lens of the objective and the object 

 when the objective is in focus. It is always less than the equivalent 

 focal length of the objective. 



Make a wooden wedge 10 cm. long which shall be exceedingly thin 

 at one end and about 20 mm. thick at the other. Place a slide on the 

 stage and some dust on the slide. Do not use a cover-glass. Focus the 

 dust carefully first with the low then with the high objective. When 

 the objective is in focus push the wedge under the objective on the slide 

 until it touches the objective. Mark the place of contact with a pencil 

 and then measure the thickness of the wedge with a rule opposite the 

 point of contact. This thickness will represent very closely the work- 

 ing distance. For measuring the thickness of the wedge at the point of 

 contact for the high objective use a steel scale ruled in £ths mm. and 

 the tripod to see the divisions. Or one may use a cover-glass measurer, 

 for determining the thickness of the wedge (Ch. VIII). 



For the higher powers, if one has a microscope in which the fine ad- 

 justment is graduated, the working distance may be readily determined 

 when the thickness of the cover-glass over the specimen is known, as 

 follows : Get the object in focus, lower the tube of the microscope un- 



