Ch. II] WORKING DISTANCE WITH THE MICROSCOPE 41 



Determination of Working Distance 



§ 78. Working distance, no cover. — As stated in § 77, this is 

 the distance between the front lens or mounting of 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 or an ink or pencil mark on the slide. Do not 

 use a cover-glass. Use a 16 mm. objective and focus the dust or mark 

 carefully, and when the objective is in focus push the wedge between 

 the objective and 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 working distance. For measuring the 

 thickness of the wedge at the point of contact for the high objective 

 use a steel scale ruled in 5 mm. and the tripod magnifier to see the 

 divisions. Or one may use a cover-glass measurer (Ch. X) for deter- 

 mining 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 deter- 

 mined as follows: 



Use the marked slide as above. Get the dust or mark in focus, 

 then lower the tube of the microscope until the front of the objective 

 just touches the slide. Note the position of the micrometer screw and 

 slowly focus up with the fine adjustment until the dust or mark is 

 again in focus. By noting the total and partial revolutions of the 

 graduated fine adjustment the working distance will be known. For 

 example, suppose it required 5.5 revolutions of the micrometer screw 

 to raise the objective from the surface of the slide where the object 

 is located to a point where the microscope is in focus, and the mi- 

 crometer screw raises the objective 0.1 mm. for each complete revolu- 

 tion, then the total elevation will be 0.1 X 5.5= 0.55 mm., that is, the 

 working distance in this case is 0.55 millimeter. 



§ 79. Free working distance in covered objects. — Use a 4 mm. 

 objective and the fly's wing or any covered object. Set the fine ad- 



