50 CORRESPONDENCE. 
of varnish. Within a few months past it has been brought forward 
by Rev. John Bramhall, of Lynn, England ; its previous use and 
publication having either escaped the notice or slipped from the 
memory of himself as well as of the distinguished microscopist who 
has indorsed it and proposed to name it after him.” 
CORRESPONDENCE. 
Mr. Inapren’s STANDARD OF MEASUREMENT OF MAGNIFYING 
Power. 
To the Editor of the ‘ Monthly Microscopical Journal.’ 
Srr,—I did not think that the description of my exhibit at the 
last Scientific Meeting of the Royal Microscopical Society would be 
printed, or I would have stated the general principle of my proposed 
standard for measuring the magnifying power of microscopes. My 
note referred only to the instrument exhibited. The principle is as 
follows :—A diaphragm of any convenient size is placed at a dis- 
tance of ten times its diameter below the surface of a slip of glass in 
the position of the object on the stage. The diameter of the dimi- 
nished image of this diaphragm, as seen above the eye-piece, is 
measured. Then 
diam. of image ~ the magnifying power employed, 
referred to a distance, or length of body equal to ten times the unit 
of measurement. Thus, if the unit be an inch, the power is referred 
to a standard of 10 inches; a larger or smaller unit increases or 
diminishes the standard of power rateably, but is easily converted 
into a 10-inch standard. The size of the diaphragm is immaterial, 
provided that it is placed at ten times its diameter below the focus 
of the objective, and a slit or scale can also be used. The measure- 
ment of the diminished image is easily effected by means of a second 
microscope, having an eye-piece micrometer of known value, placed 
in a line with, and looking into the eye-piece of the first microscope, 
both being horizontal. 
I remain, Sir, yours obediently, 
Joun E. Ineprn. 
