296 E. M. NELSON ON THE MEASUREMENT OF THE 



equivalent focus of a microscope objective, can be quickly and 

 easily measured. 



The apparatus required is a stage micrometer and a screw 

 micrometer with a positive eye-piece. With a tube of a length 

 as described below, the interval of two divisions of the micro- 

 meter 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 \/ - -f- 0335, where p is the nominal initial power. Example: 

 V 



The initial power of a half-inch is required. The nominal 

 power of a half-inch is 20, which is p, then 15 A/ + 0*335 = 



15v/0-385 = 15 x 0'62 = 9*3 inches tube length. 



The tube must be drawn out until the web is 9 - 3 inches from the 

 nose-piece, and, with the half-inch on the nose-piece, two y^^ths 

 of an inch divisions on the stage micrometer are spanned by the 

 webs. The drum then is read, say, 22*4, and this is the initial 

 power of that half-inch, without any further calculation; its focal 



length is ip^ ov Q'^Afi inch. 



In case the nominal initial power is unknown, it is first deter- 

 mined with, say, a 9^-inch tube, the value thus found is inserted 

 in the equation and the measurement made again with the 

 correct tube length. All powers of quarter-inch and less focus, all 

 Zeiss's apochromats of whatever focus, and other makers' apoch- 

 romats, require a 9-inch tube. 



