MICROMETRY. 137 



determine the number of spaces on the stage micrometer which the 

 100 divisions of the ocular micrometer cover. To distinguish the 

 ruling of the ocular from that of the stage micrometer, revolve the 

 ocular with the fingers. 



The tube length which is used at the time of standardizing must 

 always be adhered to in subsequent measurements. 



Example: With a 2/3-in. objective, the 100 rulings of the ocular 

 fill in 15 of the i/io millimeter rulings (ioo//) and 3 of the i/ioo 

 millimeter spaces (lOfi). Consequently the 100 spaces of the ocular 

 cover 1530 mikrons (15 x 100 = 1500; 3 x 10 = 30). Then if 100 

 spaces equal 1530 mikrons, one space would equal 15.3 mikrons. With 

 the i/6-in. objective the 100 ocular spaces would cover about 3 of the 

 i/ 10 millimeter (ioo ( ) spaces of the stage micrometer. Then the 

 100 spaces would equal 300 mikrons and one space would equal 3 

 mikrons. 



The ruling of the slide of a Thoma Zeiss haemacytometer will 

 answer as well as a stage micrometer. The small squares are 1/20 of a 

 millimeter square, consequently the distance between the lines border- 

 ing the small square is 1/20 millimeter or 50 mikrons. 



Now, if wifh the 1/6 in. objective, the 100 lines on the ocular fill 

 in the spaces of 6 small squares, the length of such a space would be 

 50 x 6 = 300 mikrons. This divided by 100 spaces would equal 3 ,. 



The most accurate instrument for measuring is the filar microm- 

 eter. These are expensive. Measurements can also be made with 

 the camera lucida, but it takes considerable time to make the adjust- 

 ments necessary, so that it is not convenient. With an ocular microm- 

 eter one can make measurements of blood-cells, amoebae, etc., in a 

 few seconds it only being necessary to slip in the ocular micrometer. 



Rule for determining the magnifying power of microscopic lenses: 

 Measure the diameter of the lens of the objective in inches the 

 approximate equivalent focal distance is about twice the diameter. 

 Dividing 10 by the equivalent focal distance gives the magnifying 

 power of the lens. This should be multiplied by the number of times 

 the ocular magnifies. Example: The diameter of the lens of the 

 objective was found to measure 1/2 in., the focal distance would 

 then be about i inch. Dividing 10 by i we have 10 as the magnifying 



