Microscopical Objectives and Oculars. 
17 
tions most nearly representing those powers. But even if this 
were done, and much more now when this is certainly not done, or 
not done upon such a uniform plan as to be satisfactory, microsco- 
pists should always re-examine their lenses, in order to be definitely 
informed in regard to one of their most important properties. 
The easiest method of examining the magnifying power of an 
objective, by measuring the image (of a known object) which it 
forms at a standard distance (now 10 inches), was as well under- 
stood a hundred years ago as now ; a lattice of fine silver wire or 
of human hair, or a scale ruled on glass, being used to measure the 
image.* A positive ocular f or the eye-lens of a negative one is 
used as a simple microscope with which to read off the measure- 
ment. If a separate piece of apparatus were to be made for the 
purpose of measuring these powers, a positive ocular, with micro- 
meter attached, would doubtless be preferred, it being placed by 
means of the draw-tube or some other contrivance at such a height 
that its micrometer should be 10 inches from the objective. Its 
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 a 
stage micrometer, would give the magnifying power of the objec- 
tive. But as few are possessed of a large variety of apparatus, or 
care to buy a piece for so infrequent a use as this, the measurement 
is generally made with an arrangement which every microscope 
ought to include — a negative ocular, with a micrometer in the 
focus of its eye-lens, whose advantages for general micrometry are 
so well understood, giving the best view of the object and a suffi- 
ciently good view of the measuring lines, that it is usually preferred 
for that purpose. Of course the field lens is removed in measuring 
the power of the objective alone, $ but replaced for ordinary work. 
If it should be thought best to name lenses by their magnifying 
power alone, the power ascertained could be at once attached to the 
lens, the present 1-inch lens becoming No. 10, or x 10 :§ but if it 
should be the usage to name it by its power when combined with 
some standard (say 2-inch) ocular, it would be marked No. 50, or 
x 50, or perhaps x 45 or x 55. Should it be preferred to retain 
the nomenclature by inches of focal length, a power of ten dia- 
* The measurement of the image, formed by the objective only, on a screen 
at a distance of several feet, as employed by Dr. J. J. Woodward at the Army 
Medical Museum at Washington, is unquestionably the most reliable method of 
determining the amplifying power; but is a method which requires too many 
applications and too much skill to be universally applicable. 
f The convenience and growing popularity in America of this Continental 
term suggest the propriety of its general substitution for the awkward name eye- 
piece. 
X Dr. J. J. Higgins, in the ‘American Naturalist,’ Dec. 1870, p. 628. 
§ It might be 9 or 11, and thus the various degrees of power would be con- 
veniently expressed. 
