2 A. A. MERLIN ON RESOLUTION OF AMPHIPLEURA PELLUCIDA, ETC. 



It was found, however, that in actual practice the 4 mm., 

 \ised in conjunction with a 27 compensating ocular, with which 

 eyepiece the image remained perfectly sharp, would steadily show 

 the fine transverse striae on realgar mounts, although the lineation 

 was much fainter than that revealed by oil-immersion lenses 

 of large aperture. 



The resolution of valves in realgar ha\dng been accomplished, 

 dry and balsamed specimens were next examined, and to my very 

 considerable surprise, both proved resolvable with the 4 mm. and 

 l^ths axial cone. In balsam the striae appear as extremely faint, 

 but clean, grey lines of great fineness. Although most faint and 

 difficult, they have been held with perfect certainty for short 

 intervals, slightly averted vision proving of material assistance in 

 this instance. 



In order to satisfy myself that the true striae are indeed 

 rendered visible by the 4 mm., a valve has been first arranged 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 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 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 ; hence it 

 would appear that the Zeiss 4 mm. of N".A. • 95 (nominal), 

 illuminated by a fths solid axial cone, is in practice capable of 

 revealing structure just within the theoretical resolving limit of 

 a lens of N.A. •96. and 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 possess 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 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 

 extraordinary and interesting result, it having been hitherto 



