379 



NOTES. 

 ON THE MEASUREMENT OF MAGNIFYING POWER. 



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

 {Read in title, January Sth, 1918.) 



The first duty of every one who lias purchased a microscope is ta 

 determine the combined magnifying power of its various object- 

 glasses and eye-pieces. The first three extras to be purchased 

 should be a stage micrometer, a micrometer to drop into an eye- 

 piece, and some sort of camera for drawing. The cost of these 

 being about 6s. each, the outlay required is small, but it should 

 be known that these three things are " essentials " if real micro-- 

 scope work in any branch is to be undertaken. 



On a previous occasion (June 24th, 1913) I brought to your 

 notice a method of measuring the magnifying power of a micro- 

 scope, June 1913 (Ser. 2, Vol. XII., No. 73, p. 239), and now I 

 propose to lay before you a better plan. 



As the measurement of the various objects upon which one is at 

 work is a most important microscopical duty, it is necessary to- 

 know the value of the divisions in the stage micrometer in terms 

 of the eye- piece micrometer. These values for all the objectives 

 in the microscopist's battery should be entered on a card and 

 kept with the microscope in its box. This is the simplest of all 

 microscope measurements, for it consists merely in finding out 

 how many divisions of the eye- piece micrometer are required to 

 span O'l mm. on the stage micrometer. When these values are 

 known the size of any object can be at once calculated. I have 

 published a small table * to save the microscopist the trouble 

 of this calculation. In that table these values, viz. those of the 



* Micrometric Table by Edward M. Nelson. H. F. Angus & Co,,. 

 83, Wigmore St., London, W. (1914). 3d. 



