MAGNIFICATION 75 



among makers in the designation of their eyepieces, 

 some marking them with numbers, others with letters 

 or with their equivalent foci, and others, again, with 

 a magnifying power. Reference must accordingly be 

 made to their catalogues for information as to the power 

 of the respective eyepieces. Those marked A, B, C, etc., 

 and 1, 2, 3, etc., are always in ascending order of magni- 

 fication, but the same intervals between each are not 

 used by all makers who adopt one or the other notation. 

 Even when magnifying power is marked, the information 

 is not of great use unless the tube length for which 

 the maker in question adjusts objectives, and the con- 

 vention he adopts for expressing their initial power, are 

 known. 



Magnification. The magnification produced in a 

 photomicrograph is determined most accurately by 

 replacing the object, after exposure has been made, 

 by a stage micrometer, or slide on which lines T^ &nd 

 ToW of an inch, or T V and T w of a mm. apart are ruled, 

 and measuring the intervals between the lines when 

 focussed on the ground glass. This should always be 

 done, when exact measurements are required, before 

 any alteration is made in the camera extension or other 

 parts of the apparatus. Sliding calipers with verniers 

 reading to T V mm., or roV', are very useful for measuring 

 the distance between the lines on the ground glass, and 

 the mean of several divisions should be taken to avoid 

 slight errors in graduations on the micrometer. The 

 magnification produced is the quotient of the measured 

 distance between any number of lines and their actual 

 separation on the micrometer. Thus, if three of the TW" 

 graduations cover 4-2" on the ground glass, the magni- 

 fication is 4-2 divided by T in>, or 140 diameters. 



The magnification given by a lens is measured by 

 the distance between its upper focal plane and the 

 image plane, divided by the equivalent focal length. 



