USING THE MICROSCOPE 



373 



stage micrometer, and then find the vahie of one space in the ocular 

 micrometer. The method of finding this value is shown in the follow- 

 ing case in which the tube length was 160 mm.; the ocular, a Zeiss 

 ocular micrometer 2; and the 



.— tyel(z.na> 



oj' the Oculatf" 



Diaphragm! 

 ^^•p— Field lens ' 

 .— -DrawTube 



.--Body Tube 



objective, a Leitz 3. In the oc- 

 ular micrometer, ninety-eight 

 spaces covered just fifteen of 

 the larger spaces of the stage 

 micrometer. Since the stage 

 micrometer is ruled in tenths 

 and one-hundredths of a milli- 

 meter, the fifteen spaces equal 

 1.5 mm., or 1,500 ju-^ Then 

 ninety-eight spaces of the ocu- 

 lar micrometer equal 1,500 fx; 

 and one space in the ocular 

 equals -g'g of 1,500 m, or 15.3 m- 

 This value being determined, 

 there is no further use for the 

 stage micrometer. To measure 

 the diameter of a pollen grain 

 put the preparation on the 

 stage, using the same objective 

 and ocular micrometer, and 

 note how many spaces a pollen 

 grain covers. If the pollen grain 

 covers five spaces, its diameter 

 is five times 15.3 ix, or 76.5 n. 

 In the same way, the value of 

 a space in the ocular when 

 used with the other objectives 

 should be determined. The 

 values for three or four objec- 

 tives may be written upon an ordinary slide label and pasted upon the 

 base of the microscope for convenient reference. 



This method is the best one for measuring spores and for most 

 measurements in taxonomy. 



iQne millimeter= 1,000 At. The Greek letter ^ is an abbreviation for niKpov, 

 or micron. 



_D raw Tube Diafi'ira grfl 

 wilh Society Screw 



.SocUly Sera* 



..--•Mount 



Baoxlens v^, ^^e 



■front lens \ObjecLlVe 

 ..WorkinqDistancej 



Slid&i 



Objact 



Fig. 138.— Tube length 



