239 



NOTES 



ON A NEW METHOD OF MEASURING THE 

 MAGNIFYING POWER OF A MICROSCOPE. 



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



{Read June 2Wi, 1913.) 



Many microscopists, at one time or another, will have experienced 

 some trouble about the determination of the combined magni- 

 fying powers of their objectives and eyepieces. Some never 

 measure them at all, and rely upon the manufacturer's catalogues 

 for the results. This is not very satisfactory, for neither 

 objectives nor eyepieces turn out to be at precisely their nominal 

 foci ; and if both these should happen to be either in excess or 

 deficit the actual magnifying power will differ considerably from 

 that given in the catalogue. Therefore it will be better for every 

 one to measure the magnifying powers of the lenses of their 

 microscope. 



There are two well-known methods of doing this. The first, 

 and perhaps the simplest, is to employ a photomicrographic 

 camera to measure the magnified image of the stage micro- 

 meter when projected on to the ground glass at a distance of ten 

 inches. The second is to project the magnified image of the 

 stage micrometer, by means of some sort of a camera lucida, on to 

 a scale, distant ten inches as before. 



All this appears delightfully simple, but w^hen examined more 

 carefully it is not really so. First, the photomicrographic-camera 

 method requires a dark room, or the measurement must be made 

 at night, and of course it is sure to happen that when the 

 camera is most wanted it is not available. This is just what has 

 occurred to me. My photomicrographic camera and stand, which 



