205 



ON THE MEASUREMENT OF THE INITIAL MAGNI 

 FYING POWERS OF OBJECTIVES. 



By Edward M. Nelson, F.R.M.S. 



{Read November 25th, 1913.) 



Fig 9. 



The majority of uiicroscopists only concern themselves with the 

 total magnifying power of their microscopes, but some wishing to 

 probe further into matters want to know the initial power of their 

 objectives. 



The initial magnifying power, m, of an objective is -7-, but the 



focal length (f) of an objective is a very difficult thing to measure 

 directly. Usually it is found by an indirect method of measuring 



the magnifying power, for, as above, = /. 



Probably the best way of measuring the focal length by the 

 indirect method is to project the image of a measured object, 

 placed 100 inches from the stage, and to measure the diminished 

 image at the focal point of the objective by means of a microscope, 

 fitted with a screw micrometer ; the magnification, m, thus 

 obtained will give the focal length with great accuracy, for 



f ~ z. As the numerator is 100, the result can be found in 



nt 4- 'Ji 



a reciprocal table, without the necessity of doing a division sum. 



Simple as this seems, it is however a troublesome thing to do ; 

 but by the method here described the initial power, and hence the 



