MAGNIFYING POWER OF A MICROSCOPE. 241 



the exact diameter of the field by means of the stage micro- 

 meter. The product of these two quantities is the constant of 

 that eyepiece with the given tube length. 



Example : objective -in., eyepiece compensating x 8, tube 

 length 170 mm., measured magnifying power 280 diams. : 

 measured field 0-023 in. Product is 6'44, which is the constant 

 of that eyepiece for 170-mm. tube. 



The power of any other objective with this eyepiece and tube 

 length can be determined by merely measuring the diameter of its 

 field by the stage micrometer ; for the magnifying power will ob- 

 viously be the eyepiece constant divided by the diameter of the field. 



Thus, the problem of measuring the combined magnifying 

 power is brought down to the bed-rock of simplicity. No camera, 

 no regulation of illumination, no ten inches to measure ; in brief, 

 nothing to do but to count the number of divisions of the stage 

 micrometer in a diameter of the field and then divide this into 

 the eyepiece constant. 



Example 1. With the same x8 compensating eyepiece and 



170-mm. tube a |--in. objective gave a diameter of field of 



6-44 

 0*0165 in. The magnifying power therefore is r w = 390. 



Example 2. With the same x 8 compensating eyepiece and 



1 70-mm. tube a 1 J-in. objective gave a diameter of field of 0*185 in. 



6-44 



The magnifying power therefore is .... = 35. 



The determination of the constant is scarcely any more trouble 



than the measurement of the magnifying power of one objective, 



and when once found need not be determined again ; it would 



ndeed be most helpful if manufacturers would measure these 



constants and engrave them upon the tubes of their eyepieces. 



Obviously the diameter of the field can be measured while the 

 microscope work in hand is being carried on, for it disturbs neither 

 the microscope nor its adjustments. 



This method has been tested with thirty- three object glasses, 

 ranging from a 3-in. to a yTrth of 1'4 IS". A., by fourteen different 

 makers, and with various eyepieces, on three different microscopes 

 with different tube lengths, and it has been found correct. 



My best thanks are due to Mr. Grundy for his kind assistance and 

 notes. Further experience has shown that in determining the 

 " constant " it is better to measure the magnifying power by 



