394 SUMMARY OF CURRENT RESEARCHES RELATING TO 



MICROSCOPY. 



A. Instruments, Accessories, etc.* 

 (2") Eye-pieces and Objectives. 



Measurement of the Initial Magnifying Powers of Objectives.* 

 E. M. Nelson points out that the problem involved is merely that of 



solving the equation m = -— , but that the focal length (/) of an 



objective is a very difficult thing to measure directly. Indeed, it is 

 usually found by an indirect method of measuring the magnifying 



power, for, as above, / = — . Probably the best way of measuring the 



lit 



focal length by the indirect method is to project the image of a measured 

 object placed 100 in. 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 (in) thus obtained 

 will give the focal length with great accuracy, for / = 100 -f- (m + 2). 

 As the numerator is 100, the result can be found in a reciprocal table 

 without the necessity of doing a division sum. Simple as this seems, 

 it is, however, a troublesome thing to do ; but the author describes a 

 method by which the initial power and consequently the equivalent 

 focus of a Microscope objective can be quickly and easily measured. 



The apparatus required is a stage micrometer and a screw micro- 

 meter with a positive eye-piece. With a tube of a length as described 

 below, the interval of two divisions of the micrometer scale on the stage 

 is read on the drum of the eye-piece, and this reading will be the initial 

 magnifying power of the objective. The only difficulty here is the 

 determination of the proper tube-length. The tube-length is to be 

 measured from the web in the eye-piece to the end of the nose-piece of 

 the Microscope. 



The formula for the determination of the tube-length is 



15 x/ 1 + °' 335 



where p is the nominal initial power. Example : The initial power of 



* This subdivision contains (1) Stands ; (2) Eye-pieces and Objectives ; (3) 

 Illuminating and other Apparatus ; (4) Photomicrography ; (5) Microscopical 

 Optics and Manipulation ; (6) Miscellaneous. 



t Journ. Quekett Micr. Club, xii. (1914) pp. 295-300 (1 fig.). 



