72 Remarks on the Nomenclature of Achromatic 
In this connection I may add that the actual plan of the Con- 
tinental opticians is also unsatisfactory. In the matter of angle of 
aperture, -when they give it at all, they also, as a rule, give only the 
maximum. In the matter of magnifying power, when they give 
any information, they attempt, as a rule, to give the magnifying 
power of the objective with each eye-piece as actually looked through 
when in use. But as the magnifying power of the combination 
under these circumstances involves the distance of distinct vision for 
each observer, it is evident that the figures thus furnished cannot 
have any practical value. 
Pending the adoption of some such system as I have suggested 
above, it will be necessary for the microscopist to measure for him- 
self the magnifying power of the objectives he uses, whenever he 
desires to be possessed of the real information concealed behind all 
the several systems of nomenclature at present in use. Mr. Cross, 
in his paper, gives two modes of doing this. I myself have been 
in the habit of using a dark room, and throwing the image of the 
micrometer on a white screen, by the direct rays of the sun. Very 
nearly equal accuracy may be attained, however, by any microscopist 
who possesses an ordinary glass eye-piece micrometer and a stage 
micrometer. The glass eye-piece micrometer, it will be recollected, 
is slipped through the eye-piece in such a manner as to be just in 
the focus of its eye-glass, that is, in the plane which is occupied by 
the image formed by the objective when it is seen most distinctly. 
If now the field-glass of such an eye-piece be removed, and the stage 
micrometer carefully brought into focus, a comparison of the divi- 
sions of the eye-piece and stage micrometers will give the magni- 
fying power of the objective alone at the distance actually existing 
between the two micrometers. Thus, for example, if the stage 
micrometer is in T oVoths and the eye-piece micrometer in ^jjtlis of 
an inch, five times the number of eye-piece divisions corresponding 
to one division of the stage micrometer will be the magnifying 
power of the objective at the cover correction employed, for the 
distance selected. We may then apply the following simple rule, 
derived from the formula of Mr. Cross. 
Multiply the distance between the two micrometers in inches 
and decimals, by the magnifying power, and divide by the square 
of the magnifying power plus one ; the result will be the equivalent 
focal length (for the given conditions) in decimals of an inch. 
It will, moreover, he found in practice that for powers equivalent 
to those of a Jth or shorter focal lengths a still simpler rule may 
be adopted, and that the distance between the two micrometers, 
divided by the magnifying power, will give very nearly the same 
results as are obtained by the more complex rule, but this of course 
is not true for lower powers. 
It must, however, be constantly borne in mind, that the results 
