138 UNIFORMITY OF NOMENCLATURE IN OBJECTIVES. 
the image.* A positive ocular t or the eye-lens of a negative 
is used as a simple microscope with which to read off the measure- — 
ment. Ifa separate piece of apparatus were to be made for the 
purpose of measuring these powers, a positive ocular with microm 
eter attached would doubtless be preferred, it being placed 
means of the drawtube or some other contrivance at such a height 
that its micrometer should be ten inches from the objective. — 
reading would then give the real size of the image formed at that 
distance by the objective, and the ratio of this number to the 
known size of the object, say the distance apart of two lines on 
stage micrometer, would give the magnifying power of the objective. 
But as few are possessed of a large variety of apparatus, Or ¢ 
to buy a piece for so infrequent a use as this, the measurement! 
generally made with an arrangement which every microscope oug 
to include, a negative ocular with a micrometer in the focus of 
eye-lens, whose advantages for general micrometry are s0 
understood, giving the best view of the object and a sufi ici 
good view of the measuring lines, that it is usually preferred for 
that purpose. Of course the field lens is removed in meast 
the power of the objective alone,{ but replaced for ordinary work. 
If it should be thought best to name lenses by their magn if 
power alone, the power ascertained could be at once attached 
the lens, the present one-inch lens becoming No. 10, or x 10 
but if it should be the usage to name it by its power when com- : 
bined. with some standard (say two-inch) ocular, it pire: ; 
marked No. 50, or x 50, or perhaps x 45 or x 55. Should 
preferred to retain the nomenclature by inches of focal lens 
power of ten diameters might be called a one-inch lens, and p 
above and below rated in proportion. This plan is within 
of the opportunities of every microscopist, while the plan of 
ally employing a single lens of small aperture and exactly 0 
ocus as a standard of comparison is only adapted to the 
*The measurement of the image, formed by the objective ser on a sereen 
tance of several feet, as employed by Dr. J. J. Woodward at the Army * 
seum at pisemas n, is u Semis kika the most reliable soi of etern 
amplifying po but is a sein which requires too many a pplications ang" 
1 to be universally applic oo 
t The convenience y aoras popularity in | this country p his bom 
pe te ates the awkwa 
¢ Dr. Higgins in ae American Naturalist, Dec., 
1870, p. 628. 
roe It rt be 9 or 11, and thus the various degrees rs deat vw þe ¢ 
