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VIII. — The Penetrating Power of the Microscope. 
By Edward M. Nelson. 
(Read 18th May, 1892.) 
It has long been recognized that the penetrating power of the Micro- 
scope, or what, in other words, is the capability of seeing two different 
planes at the same time, diminishes as the magnifying power is 
increased, as the aperture is increased, and as the refractive index of 
the mounting medium is lowered. 
The late Dr. Carpenter placed an undue amount of importance on 
the penetrating quality of the Microscope, and by his erroneous 
teaching on this subject undoubtedly retarded the advance of 
microscopy. His error was twofold : — 
(1) Penetration is not an unmixed advantage in the interpretation 
of microscopical images ; it is more often a positive disadvantage. 
In objects, the nature of which cannot be perceived by the naked eye, 
it is better to trust to focal adjustment than to the best visual appear- 
ance of depth, however obtained. 
(2) A “ low-angled glass for penetration ” is not the best way of 
obtaining the desired end. 
It should be particularly noted that the difference in penetration 
between a 1/4 of 60° ( - 5 N.A.), and a 1/4 of 140° ( - 95 N.A.) both 
with an amplification of 300 diams. (B eye-piece) is less than 
1 /20,000 in. — an almost inappreciable quantity with those lenses. 
In cases where I would use a 1/2-in. of *5N.A., and an amplifica- 
tion of 150 diams. (B eye-piece), giving a penetration of 1/2500 in., 
a biologist or histologist would use a 1/6 of ’6 N.A. (Zeiss D) with 
an amplification of 450 diams. (B eve-piece) and a penetration of 
about 1/20,000 in. 
For many years the world of microscopy heard a great deal of 
Carpenter’s dictum, “ low-angled glass for penetration ” ; a little time 
therefore spent on the examination of Dr. Carpenter’s tables of powers 
and apertures in the ninth edition of the ‘ Encyclopaedia Britannica ’ * 
will not be amiss. 
We find here a H-in. of 20° and a 1-in. of 30°, powers 54 and 
75 respectively ; these are full apertures required by the most critical 
workers for those amplifications.! No one could tell the difference in 
an image amplified 75 diams., with an apochromatic inch of ‘3 N.A. 
stopped down to '26 N.A., or 1/2-in. of - 65 N. A., which fact demon- 
strates that there would be no gain in increasing the aperture of 
the inch beyond ’26 N.A. with that amplification. As I stated 
in the ‘English Mechanic’ (1883), before the introduction of apo- 
* Vol. xvi. p. 270. 
t An apochromatic li of ' 18 or -2 N.A. is a want much felt. 
