A PLEA FOR WIDE APERTURES. 235 
working distance be lost. We can readily explain our views by 
citing several cases in point. 
The longest working distance in a half-inch objective with which 
we are acquainted is 0.25 inch, with an air angle of 39°. Let us 
take this as our standard (but, perhaps, these figures can be 
improved upon), and in doing so assume that it is advisable to stop 
enlarging the angle of the one-inch objective directly the working 
distance runs level with 0.25 inch. 
By using this argument, it will be seen that it is possible to 
manufacture lenses with too wide an aperture. In the work, “ How 
to see with the Microscope,” by Dr. J. E. Smith, a Spencer inch 
of so° is stated to have a working distance of 0.13 inch, about half 
the distance of that of the half-inch of 39°. But a half-inch of 50° 
could be made with a longer working distance than 0.13 inch, 
therefore we do not see what has been gained by enlarging the 
aperture thus far, 
We happen to possess a fine Tolles one-inch of 34°, made 
specially for us ; it has a working distance of 0.39 inch, and we have 
found 0.4 inch to be the usual working distance for nominal inch 
objectives of30°. Again, the longest working distance possessed by any 
well connected quarter-inch, which has passed through our hands, 

has been o.10 inch for 41° and Zeiss C (¢ inch) of 50° had a work- | 
ing distance of 0.06. 
Now, if it is held that the quarter-inch of 50° is a useful objective 
for the anatomist or histologist, it is clear that there would be no 
advantage in reducing the working distance of the half-inch below 
0.06; on the other hand, there is an advantage in increasing the 
angle until the working distance is reduced to 0,10 inch, 
The quarter-inch of 130° may be made with a working distance 
of 0.02 inch, and the one tenth of 140° air angle, with a working dis- 
tance of 0.01 inch, is also common, but as we are only taking the 
lower powers into consideration, the higher amplifications will not 
be more than noticed. 
For lower powers than the inch we see no reason why the 
apertures should not be considerably increased, as for all of them 
there would be sufficient room for top illumination or for working 
through a moderately thick trough. Of course, with the 2”, we are 
stopped at about 20° by the diameter of the Society screw, but this 
has been remedied by the introduction of the “ Butterfield,” or 
Broad Guage screw. 
The above are our views, and we hold that the possession of a 
set of objectives, a two-inch of 20°, or even more; a one-inch of 
35°, or even 40°, if the working distance of 0.40 inch can be 
preserved ; and a half-inch of 66° or 70°, so as to give a working 
distance of 0.10—o0.12; with a quarter-inch of 100°, and higher 
powers as necessary, will enable all kinds of work to be performed 
‘cr owe 
