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Transactions of the 
Admiral Smyth says, that “ it does not require any knowledge of 
the thickness and focal lengths of any of the lenses employed in the 
telescope, nor yet of their number and relative positions;” and he 
quotes Dr. Pearson thus: “One operation includes the result arising 
from the most complicated construction, and sets theory at defiance 
with respect to calculations that must take into consideration the 
previous determination of all the preceding requisites, the obtaining 
of which is attended with practical difficulties almost insurmount- 
able.”* I can hardly imagine but that some one interested both 
in telescopic and microscopic research must have long since applied 
the same method to the microscope that has been used for the tele- 
scope with perfect success for more than a century past. So far as 
I know, however, this has not been done, or at any rate, published, 
and I therefore venture to bring this matter before the Boyal Micro- 
scopical Society, in the hope that it may be of interest and utility to 
some few amongst us, and perhaps lead to further inquiry. 
The conditions under which we use this plan of obtaining the 
magnifying power are not quite similar in the two cases. In the 
astronomical telescope we deal with parallel rays, and tire focus is 
therefore constant for each instrument : in the microscope we use 
various objectives as well as eye pieces, and conjugate foci instead of 
parallel rays. But if we fix upon a standard object and view it at a 
known distance the method is equally applicable. The standard I 
propose to use is the diminished image of a disk one inch in dia- 
meter, placed at ten inches below the position of an object upon the 
stage of the microscope ; then the real diameter of that disk divided 
by the diameter of its reduced image will give the standard magni- 
fying power with the objective eye-piece and length of body em- 
ployed, and if the length of body is such as to make the distance 
between the front surface of the objective and the diaphragm in the 
eye-piece ten inches, then the real as well as the standard magni- 
fying power is given. 
The reduced image of a disk one inch in diameter viewed at ten 
inches below the stage is too small to be measured with accuracy 
when high powers and deep eye-pieces are used, but any multiple of 
one inch or divisor of ten inches can be employed. For most pur- 
poses the following plan seems to answer : A revolving diaphragm, 
capable of being adapted to the stage, is pierced with apertures of 
J inch, \ inch, and inch, and fixed to a tube at inches from 
a plate of thin glass, upon the upper surface of which the objective 
can be focussed. This will allow of the objective being used at “un- 
covered,” at “ covered,” with any known thickness of covering glass, 
and with a wet or dry front. On the same, or another diaphragm, 
some minute holes, at known distances apart, such as i inch and 
1 J inch, can be pierced, and used in the same manner. It is evident 
* Smith’s ‘Celestial Cycle,’ i.. 382. 
