76 PHOTOMICROGRAPHY 



The magnification produced by a microscope depends 

 on two factors : 



1. The magnification produced by the objective in 

 an image formed in the lower focal plane of the eye- 

 piece, called the " initial magnifying power " of the 

 objective, and expressed by 



A 



focal length of objective 



where A is the optical tube length, that is the distance 

 between the upper focal plane of the objective and the 

 lower focal plane of the eyepiece (see Fig. 22). 



2. The magnification of that image by the eyepiece 

 when forming the final image, supposed to exist at the 

 normal minimum distance of distinct vision from the 

 eyepoint, called the power of the eyepiece, and expressed 

 by- 



250 



focal length of eyepiece in mm. 

 or 



10 



focal length of eyepiece in inches 



There are two other factors which govern magnification 

 in photomicrography, besides the powers of the objective 

 and eyepiece, and they are : 



3. The tube length, on which will depend the initial 

 power of the objective as defined above. 



4. The camera extension, or distance between the 

 upper focal plane of the eyepiece and the ground glass. 



In practice the positions of the upper and lower 

 focal planes of the objective and eyepiece (shown in 

 Fig. 22 for definite lenses) are generally unknown to 

 the worker, and consequently it is convenient to make 

 all measurements of magnifications at some standard 

 distance from given points on the microscope. A table 



