1901) MICROSCOPICAL JOURNAL. 319 
mersion being afterwards substituted, when the lines have 
been found to be identical, and of the same fineness and 
distance apart with both objectives, the only difference 
being in the strength of the resolution afforded by them. 
The significance of the above results is at once appar- 
ent on turning to the aperture table, where we find that 
N. A. :96 is given as the limit of resolution of the A. pellu- 
cida; 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 theoretical resolving limit of a lens of N. A. -96, and 
also that this resolution is attainable not only in media 
of high refractive index, but also in balsam and with dry 
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 a N. A. of -96, oreven 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 
actually attained by a lens with strictly axial illumination, 
and on specimens mounted in media of both high and low 
refractive index, cannot but be regarded as a very extra- 
ordinary and interesting result, it having been hitherto 
considered that the transverse striae of the A. pellucida 
are in actual practice only just discoverable with the dry 
achromatic lenses of N. A. 1-0, and that only on speci- 
mens 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 to 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 
