1901] MICROSCOPICAL JOURNAL 123 
hence it would appear that the Zeiss 4 mm. of N.A. °95 
(nominal), illuminated by a 5-6ths solid axial cone, is in 
practice capable of revealing structure just within the the- 
oretical resolving limit of alens of N.A. -96, and that this 
resolution is attainable not only in media of high refrac- 
tive index, but also in balsam and withdry mounts. 
Now the 4 mm., although its guaranteed minimum N. 
A. is only -95, as a matter of fact is quite likely to pos- 
sess an N.A. of -96, or even one slightly in excess of this, 
so that theoretically, without any deduction for technical 
imperfections, it would be just capable of resolving the 
A. pellucida; but that this theoretical limit should be act- 
ually attained by a lens with strictly axial illumination, 
and on specimens mounted in media of both high and low 
refractive index, cannot be regarded as a very extraor- 
dinary and interesting result, it having been hitherto con- 
sidered that the transverse strie of the A. pellucida are 
in actual practice only just discoverable with dry achro- 
matic lenses of N.A. 1:0, and that only on the specimens 
mounted in a medium of about 2:4 refractive index when 
illuminated by oblique light in one azimuth along the valve. 
Perhaps not the least interesting and satisfactory out- 
come of these observations is the indication that a dry 
lens is capable of working its full theoretical capacity on 
balsam-mounted objects, the resolution only becoming 
more conspicuous in media of higher refractive index. 
_ In addition to the A. pellucida many other forms have 
been recently studied with the 4mm. and a 5-6ths solid 
axial cone. The most difficult structural features have not 
been seen with a lesser cone, but we do not assert that 
they may not possibly be so resolved, although the results 
of my observations have strongly inclined me to the belief 
that, with axial illumination, structure just within the ca- | 
pacity of the lens employed can only be seen with a very 
large cone. It has appeared to me that closing down the 
cone, while greatly strengthening the contrast of the 
