METHOD OF MEASURING MICROORGANISMS 71 



when Leitz objectives 3, 7 and 1/12 oil immersion are 

 used. 



The object micrometer is simply a 

 cover-glass (mounted on a slide in 

 Canada balsam) upon whose surface 

 has been ruled a scale 2 mm. in length, 

 each millimeter being divided into 100 

 equal parts, the space between each di- 

 vision therefore being equal to 0.01 mm. 



3. If 100 division lines of the 

 ocular step micrometer cover 0.01 mm. 

 of the object micrometer, then each 

 division line of the step micrometer has 

 the value 0.0001 mm. or 0.1 micron. 



These values are only accurate when 

 the draw-tube of the microscope is 

 drawn out according to the following 

 table. 



4. Using the ocular step micrometer 

 and the object micrometer, find the tube 

 length at which each objective gives a 

 definite value in microns. This will vary 

 some even with the Leitz oculars and 



objectives, so the tube length for each FIG. 25. Micrometer 

 combination of lenses must be deter- 

 mined separately for any make of 

 microscope. 



-:JO 



40 



70 



80 



Scale in Ocular of 

 the Leitz Ocular 

 Step Micrometer. 



VALUES FOR LEITZ ACHROMATIC OBJECTIVES 



5. Multiply the number of division lines of the ocular 

 micrometer covered by the organism in question by the value 



