A. A. MERLIN ON RESOLUTION OF AMPHIPLEURA PELLUCIDA, ETC. 3 



considered that the transverse striae of the A. pellucida are in 

 actual practice only just discoverable with dry achromatic lenses 

 of IsT.A. 1*0, and that only on 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 outcome 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 been 

 recently studied with the 4 mm. and a |^ths solid axial cone. 

 The most difficult structural features have not been seen with 

 a lesser cone, but I 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 capacity 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 coarser, causes the finer detail to disappear 

 altogether, and materially reduces the separating power of 

 the objective. With reference to this matter the following 

 experiment may prove interesting : — Arrange a Cherryfield 

 Navicula rhomboides, mounted in a mixture of monobromide 

 of naphthaline and balsam, under a good semi-apochromatic ^'' 

 of N.A. '77, and 27 ocular, so that the valve shall lie longi- 

 tudinally along and on the sharply focussed edge of the lamp 

 flame. With slightly under | cone the longitudinal striae will 

 appear conspicuous throughout the entire length of the valve, 

 while the closer transverse striae, although they may be seen 

 to a certain extent, are far less satisfactorily defined, no 

 thoroughly clear separation being apparent. Now replace the 

 smaller by a fths cone. The coarse strongly-defined longitudinal 

 striae disappear, and at the first glance all structure may seem 

 to have disappeared with them, but a little careful scrutiny will 

 reveal the presence of a faint dotted resolution, the transverse 

 divisions of which are as fully and cleanly shown as the 

 longitudinal. 



I am aware that the results dealt with in this paper cannot 



