SUMMARY OF CURRENT RESEARCHES RELATING TO 
114 
On the Amplifying Power of Objectives and Oculars in the Com- 
pound Microscope.*— Dr. G. E. Blackham writes : — “ A great deal has 
been said and written on this subject, and still the matter is not as 
clear and accurate as could be desired. 
The European opticians usually name their objectives and oculars 
in an arbitrary manner, as No. I., No. II., No. III., &c., or A, B, C, &c., 
but these designations give no clue to the amplifying powers, except 
that the lower numbers or earlier letters usually indicate the lower 
magnifying powers. The No. I. objective of one maker does not, how- 
ever, necessarily correspond with the No. I. of another maker, and the 
No. I. objective does not necessarily correspond in amplifying power 
with the No. I. ocular of the same maker. 
The English makers have attempted to avoid this confusion and, to 
introduce a degree of uniformity, have long adopted a system of nomen- 
clature based upon the amplifying power; that is, if a combination of 
lenses magnifies equal to a single convex lens of one-inch focus, the 
combination is called a one-inch ; if the same as a single lens of one- 
quarter inch focus, it is called a quarter-inch, &c., &c. This system has 
long been in use in England and this country for objectives, and more 
recently has been extended to oculars (or eye-pieces, as they are com- 
monly called). This was supposed to give a very simple and accurate 
means for determining the power of any objective or ocular or com- 
bination of objective and ocular, provided only that they were correctly 
named by the maker and were used on a tube of the standard length. 
The rule commonly in use is based upon the assumption of the arbitrary 
distance of ten inches as the distance of distinct vision, and that the 
number of times the focal length is contained in ten inches is the ampli- 
fying power ; so that a one-inch lens would magnify ten times, a one- 
fourth inch forty times, a one-tenth inch one hundred times, &c., &c. 
The image of the object projected by the objective being again magnified 
by the ocular, it was further assumed that the same rule would apply, 
and therefore that the amplification produced by the combination of a 
1/10 objective with a one-inch ocular would be foimd by multiplying 
the assumed power of the 1/10 objective (100) by the assumed power 
of the one-inch ocular (10) = 1000 diameters. And so, by the appli- 
cation of this simple rule, every owner or user of objectives and oculars 
of the new nomenclature could calculate correctly the theoretical power 
of each and of any combination, with the understanding that, if the 
distance between the optical centre of the objective and that of the ocular 
varied, the amplifying power would vary in proportion. 
The object of the present paper is, 1st, to show the incorrectness of 
this rule, in that the real image projected by a simple convex ten inches 
from its optical centre is not amplified the number of times the focal 
length is contained in ten inches, and that the same rule of amplification 
that is true and correct for the objective that projects a real image cannot 
be true and correct for the ocular, which projects a virtual image ; and, 
2nd, to present a correct method of determining the (linear) amplifying 
power of any objective or ocular, correctly named on the equivalent 
focal length system, and of any combination of such objective and ocular 
at any given distance between their optical centres. 
* Proc. Amer. Soc. Microscopists, xi. (1889) pp. 22-31. 
