122. THE AMERICAN MONTHLY (May 
ified optical arrangement through having remarked the 
great strength of the resolution yielded by some realgar- 
mounted specimens under the Zeiss 3 mm. of N.A. 1:4 and 
a solid axial cone of about N.A. 1°2 from an oil-immer- 
sion condenser. I must confess that the exact theoreti- 
cal resolving limit of an object glass of N.A. 95, as given 
in the table on page 85 of Carpenter’s ‘‘The Microscope 
and its Revelations” (Seventh Edition, Edited by Dallin- 
ger, 1891), had at the time escaped my memory, other- 
wise it is extremely improbable that any such attempt 
would have been made. 
It was found, however, that in actual practice the 4mm., 
- used in conjunction with a 27 compensating ocular, with 
which eyepiece the image remained perfectly sharp, would 
steadily show the fine transverse strix# on realger mounts, 
although the lineation was much fainter than that reveal- 
ed by oil-immersion lenses of large aperture. © 
The resolution of valves in realgar having been accom- 
plished, dry and balsamed specimens were next examin- 
ed, and to my very considerable surprise, both proved re- 
solvable with the 4 mm. and 5-6ths axial cone. In balsam 
the strie appeared as extremely faint, but clean, gray lines 
of great fineness. Although most faint and difficult,they 
have been held with perfect certainty for short intervals, 
slightly averted vision proving of materia] assistance in 
this instance. 
In order to satisfy myself that the true striw are indeed 
rendered visible by the 4 mm., a valve has been first ar- 
ranged to exhibit them under that lens, an oil-immersion 
being afterwards substituted, when the lines have been 
found to be identical, and of the same fineness and dis- 
tance apart with both objectives, the only difference be- 
ing in the strength of the resolution afforded by them. 
The significance of the above results is at once apparent 
on turning to the aperture table, where we find that N.A. 
‘96 is given as the limit of resolution of the A. pellucida ; 
