A PLEA FOR WIDE APERTURES. , 231 

There is much in the paper, already mentioned, to interest both 
the student and the professed microscopist, the inferences so 
generally stated and defined, are perhaps the weakest points in the 
communication, and it would have been much better had reasons 
been given, on a histological basis, for the selection of certain aper- 
tures for definite focal lengths, and the various researches in detail, 
for which these apertures and amplifications were found most use- 
ful. It is true that in his second paper Prof. Abbe gives the 
amplifications necessary to utilize each “aperture,” but the latitude 
allowed by the table, taken with the fact that the figures do not at 
all agree with his views as carried into practical effect by his 
brother-in-law, Zeiss, is the best argument for its total rejection. 
Having these views before us, and agreeing in great part with them, 
we have three main questions, differences of opinion, to occupy 
our attention. 
1. Are apertures, wider than those Prof. Abbe has indicated in 
his second paper, more useful than narrow ones, and why? 
2. Can apertures be reduced by means of a diaphragm (fixed or 
otherwise) without spoiling the performance of the objective? and 
3. Can penetration and long working distance be secured from a 
series of wide aperture objectives ? 
Now, if we return to the first question, referring on our way to 
Prof. Abbe’s paper, we shall find that the latitude he allows in the 
amplifications for each definite aperture stultifies any attempt to fix 
within a few degrees what the definite angle should be for a speci- 
fied objective. On page 462* he writes, “ It may be inferred from 
this example, in accordance with many similar facts, that satisfac- 
tory observation requires that the smallest detail of the micro- 
scopical image shall be displayed under a visual angle of not less 
than 2’ and not more than 4’ approximately: angles which 
correspond very nearly to the amplifications 300 and 600 for 
dimensions of 0°5 p. 
This is for an aperture of 0.6 (73°.6 in air), which shows the 
strie on P. angulatum, but nothing more, and is intended to 
illustrate what “empty amplification” is (Ze the point at which 
enlargement of the image opens out no new detail). But why 
should there be any “empty amplification?” or, in other words, 
why should not the chance of it be reduced to a minimum? Why 
should not the objective be capable of showing out detail with each 
increment of ocular so long as the quality of the image is preserved, 
the working distance regarded, the aperture capable of being 
reduced when desired, and penetration possible with suitable 
manipulation. 
To say that the microscopical image shall be displayed under a 

*J.R.M.S. 
