CH. 77] LIGHTING AND FOCUSING 47 



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 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. 



87. Determination of Working Distance. As stated in 

 69, 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 

 equivelent 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 ob- 

 jective. When the objective is in focus push the wedge under the 

 objective on the slide untifit 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 working distance. For measuring 

 the thickness of the wedge at the point of contact for the high ob- 

 jective use a steel scale ruled in 4- mm. and the tripod to see the di- 

 visions. Or one may use a cover-glass measure (Ch. VIII) for de- 

 termining the thickness of the wedge. 



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

 adjustment is graduated, the working distance may be readily de- 

 termined 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, until the front of the objective just touches the 

 cover-glass. Note the position of the micrometer screw and slowly 

 focus up with the fine adjustment until the object js in focus. The 

 distance the objective was raised plus the thickness of the cover- 

 glass represents the working distance. For example, a 3 mm. ob- 

 jective after being brought in contact with the cover-glass was 

 raised by the fine adjustment a distance represented by 16 of the 

 divisions on the head of the micrometer screw. Each division rep- 

 resented o.oi mm., consequently the objective was raised o. 16 mm. 



