' 
238 THE NORTHERN MICROSCOPIST. 

We have now possibly overcome the objection that no one would 
force 100 diameters from an inch objective when he could get a 
better picture from a half-inch, and, therefore, may now consider 
the best way to obtain penetration and long working distance from 
wide aperture objectives. 
Both oculars and objectives must be used with intelligence, and 
the question often crops up during an investigation, which shall be 
used to get a certain amplification, say of 100 diameters, the half- 
inch and the A eyepiece or the one-inch and the C ocular? 
The answer to such a question must depend upon two circum- 
stances—the working distance required, and the necessary angle. 
Is penetration required? If so, the aperture must be small, or, is the 
preparation flat and thin, and resolution rather than depth of focus 
necessary? If this be the case use wide apertures. 
Now let us illustrate the case by the half-inch objective. There 
are many lenses of this power in use possessing an aperture of 40°, 
used especially for the study of Foraminifera, Polycistina, and such 
like solid objects. Now we argue that the half-inch of 40° is 
entirely unnecessary ; a better image with more penetration, more 
light, and better definition may be secured by using the one-inch 
of 35° and the C eyepiece, and thus giving considerably more 
working distance than could ever be obtained from a half-inch. 
In taking pictures A and D the amplifications were 128 in each 
case, the working distance in A was 0.38 inch, while that of D was 
only 0.14 inch. 
If a small aperture is needed, and working distance not a sine 
qua non, the aperture may be reduced by means of the iris 
shutter as has been shown by pictures B and C, and we hope to 
have practically demonstrated the untenable position taken up by 
Prof. Abbe and his colleagues, the Editors of the Journal of the 
Royal Microscopical Society, by the accomplishment of that which 
they have been pleased to state was impossible. 
There is yet another point to be considered. Prof. Abbe raises 
an objection to wide apertures on account of the trouble the cover 
adjustment entails. We have never found any difficulty in making 
this adjustment quickly, and we are of opinion that it should be 
more studied than it is, for it isa lamentable fact that students 
know practically nothing as to how the adjustments should be made 
even after passing through a Biological laboratory, where they 
should be taught something concerning the manipulations of the 
instrument. 
We have now replied to the several objections raised against 
wide apertures, we have found much benefit ourselves in finally 
selecting objectives of the apertures here indicated, and if we have 
assisted any of our readers inthis somewhat intricate and obscure 
question we shall consider we have not written in vain. 
~— a 

