38 P.VRFOCAL OCULARS AND OBJECTIVES [Ch. II 



the fine adjustment (fig. 25), first one way and then the other, 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 

 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 

 object in the small field (§ 47, fig. 29 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 (fig. 61). 



§ 73. 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. It may be necessary to 

 refocus with the fine adjustment every time a different part is brought 

 into the field. In practical work one hand is kept on the fine adjust- 

 ment constantly, and the focus is continually varied. 



§ 74. Parfocal oculars and focusing. — On changing the oculars 

 from a higher to a lower or the reverse it is necessary to refocus the 

 microscope. Formerly the change in focus was very marked in chang- 

 ing from one power of ocular to another, but since Mr. Pennock 

 introduced parfocal oculars (1881) and their almost universal adoption 

 since, very little change in focus is necessary in passing from power to 

 power of ocular (see Ch. IX). 



§ 75. Parfocal objectives. — These are groups of objectives, of 

 different power, so mounted that when screwed into the revolving 

 nose-piece of the microscope very little change in focusing is necessary 

 in passing from objective to objective. This arrangement of objectives 

 was a natural outgrowth from the parfocalization of the oculars 



(§ 74)- 



In case the objectives are not nearly enough parfocal so that the 

 object is visible in turning from one objective to another, the defect 



