Microscopical Objectives and Oculars. 
21 
of the natural object ; but, aside from the inconvenience of the 
large and often incomprehensible numbers thus obtained, this 
method gives in one sense the magnifying power, but in no sense 
the microscopical power employed. The power to see small things 
depends, so far as real or apparent size is concerned, on the distance 
from each other of minute points of structure, and this is in the 
exact ratio of the linear magnifying power. Squaring or cubing 
this power has acquired a suspicion of sensationism, if not of char- 
latanism, and is generally avoided in science. 
If anything could he more confused and confusing than the 
different real and nominal powers of the objectives, it would be the 
corresponding powers of the eye-pieces or oculars. Made without 
any pretence of uniformity, and named without any serious attempt 
at significance, it has seemed until recently that no escape from the 
confusion was to he looked for. Yet it would seem to be con- 
venient and altogether unobjectionable to have the oculars so named 
as to express their magnifying power, and the practice of doing 
this has been already introduced into this country. Some micro- 
scopists have renamed their oculars by their magnifying power, on 
the basis of one inch to ten diameters, and I am informed by Mr. 
Bicknell that Tolies has already adopted the same plan, in naming 
those of his manufacture, discarding the letter nomenclature 
(A, B, C, &c.) and selecting 2 in., H in., 1 in., fin., ^ in., to in., 
and £ in., giving powers of 5, 7^, 10, 15, 20, 30, and 40 diameters. 
The writer has applied the same names to his oculars, applying the 
intermediate fractions y*, inch, § inch, and 1 inch, to intermediate 
powers ; and he is satisfied, by experience of its convenience, that 
this nomenclature only needs a trial, to secure its adoption by all 
who use the microscope for other purposes than amusement. Of 
course any microscopist, having determined the power of an ob- 
jective and the powers of the microscope when that objective is 
used with his various oculars, can obtain the powers of his oculars 
by dividing the latter numbers by the one first named, and can 
then name his oculars, like the objectives, either by their magni- 
fying powers or by their equivalent focal lengths. The rivalry of 
makers and the interests of trade are not involved in this case as in 
that of the objectives, and there may be no reason why this plan, if 
as acceptable to microscopists generally, as it has been to a few, 
should not come into immediate use. 
In order to work the objectives and oculars at their standard 
powers they should be, of course, about 10 inches apart, either by 
length of compound body or by use of draw-tube ; and it is believed 
that most objectives whose corrections are accurate enough to show 
any difference will work best at about this distance. Should a 
decidedly different distance he used in any observations of import- 
ance, it would be well to state that fact in recording the observation. 
