234 THE MICROSCOPICAL NEWS. 
fitted with the Society thread to allow a 2-inch objective to be 
screwed in. This combination is made to slide in another tube 
furnished with a set screw, in order that the inner tube carrying 
the optical portion may be fixed in any required position. 
In use, the outer tube is placed in contact with the glass, and 
the inner tube carrying the ocular and objective withdrawn until 
the cross lines on the glass, made with the diamond, are exactly in 
focus. When the focus is accurately obtained the set screw is 
tightened, and it follows that when the lower end of the outer tube 
is placed over the transparent circles, the sharpest image must be 
in the same plane as the diamond scratches, when its details are 
best seen with the auxiliary microscope. 
We can scarcely imagine a simpler or more accurate method 
than the foregoing, nevertheless, some may object to it on the 
ground that an auxiliary microscope is required, and therefore 
another method is given, which, if not so handy or so accurate as 
that already described, has the merit at least of being inexpensive. 
A focusing slide is used, in addition to the ground glass prepared 
as before described, and this slide is pierced with a series of holes 
to take an ordinary ocular, the A preferably. The first step is to 
secure the best focus on the transparent circle, ¢o the unarded eye, 
and the proboscis of the blow-fly will be the best object to work 
with. When this is obtained, set the eye-piece in the position of 
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Fig. 59. 
sharpest focus in the focusing slide, and always use it in that posi- 
tion, which can be ensured by a collar of sufficient depth fitting up 
to the shoulder. 
This second method was first described in “ Practical Micros- 
copy,” and Mr. G. J. Johnson’s method of accomplishing the same 
end may be found on page 116 of the present volume. 
For the benefit of those who wish to practise this art, we repro- 
duce the illustration (fig. 59) showing the focusing slide at the 
extreme left of the base board. 
