252 THE MICROSCOPICAL NEWS. 
_ THE RELATION OF APERTURE AND POWER IN 
THE MICROSCOPE. 
By Proressor E. AspBE, Hon. F.R.M.S. 
(Read 14th June, 1882). 
IIl.—The Rational Balance of Aperture and Power. 
(it.) Division of the Entire Process of the Microscope between 
Ocular and Objective. 
(Continued from page 232.) 
HE conclusion from the foregoing experimental facts must 
therefore be :— 
In order to obtain the best possible conditions for the utilization of 
the delineating capacity of any aperture, the focal lengths of the 
objectives must be sufficient to yield those powers which are necessary 
for distinct vision of the least details, with no greater super-amplifica- 
tion than ts indicated by the critical valves of v defined above. 
We shall therefore arrive at the point which is the aim of the 
whole discussion—the determination of the proper focal lengths 
for the various apertures—by tracing the practical inferences from 
this principle. 
(1) The maximum apertures of the various systems—dry, water- 
immersion, homogeneous-immersion (for crown glass)—which are 
fit for ordinary use, may be approximately assigned by the numbers 
a=0o*9Oo a=1°*20 a=1°35 
because apertures which approach the ultimate limit of any system 
by less than about ro per cent. cannot at all events be satisfactorily 
used for regular scientific work. ‘The critical valves of v for these 
apertures may be put, as has been pointed out, 
v=4 yw yv=6 
The total powers which are wecessary for the proper utilization 
of the same apertures are shown by the fourth column of the first 
table (Vol. II. 1882, p. 463), inasmuch as no observer of normal 
eyesight will be able to recognize déstznctly details under a smaller 
visual angle than 2’ of arc. Adopting the figures of the table in 
round numbers, we obtain therefore the normal amplification [N] 
which is required for the wide-angled objectives of the various 
systems— 
480 640 720 
—— = 120 —— = 160 —— = 120 
4 4 6 
