192 PROCEEDINGS OF THE 



Amphipleura pellucida was no longer a sufficient test for objec- 

 tives of the highest power : for many years any tolerable oil- 

 immersion of moderate aperture could easily reveal its once; 

 elusive transverse striae by oblique light. Really good objec- 

 tives of rS N.A. will reveal them with axial illumination. Of 

 course, to show the dotted structure is a more difficult task.. 

 " With Nitzschia singalensis the case is different, and any micro- 

 scopist who can clearly reveal the transverse striation may^ 

 feel satisfied with his optical appliances." A very beautiful 

 photograph at 3,600 diameters taken with a l/8th apochromat- 

 of 1'42 N.A., by means of oblique sunlight from a heliostat,, 

 accompanied the paper. This not only showed distinctly the 

 transverse striae, but was a most artistic production, and wa& 

 greatly admired when passed round for the inspection of the 

 members. 



In response to the President's request for a discussion on th& 

 subject, Mr. Ainslie said he quite agreed with Mr. Merlin that 

 A. pellucida as a test leaves something to be desired, and that 

 its resolution is no great feat at the present day. But as a- 

 result of his experience he thought that until we have Nitzschia. 

 mounted in realgar the resolution of the transverse striae of 

 Amphipleura with a full solid axial cone of illumination remains 

 the best test of the all-round capacities of an oil-immersion 

 objective. He considered that there is no doubt that Mr. Merlin 

 possessed unusually keen eye-sight, and thought that most micro- 

 scopists need not be surprised nor disappointed if they fail to- 

 see the striae of Nitzschia except under the most favourable 

 conditions of illumination and manipulation. Mr. Ainslie had 

 a microscope on the table, with a Leitz 1/lOth oil-immersion of 

 1'33 N.A., with which he was easily able to show the striae of 

 Nitzschia. He said not only was this interesting in connection 

 with Mr. Merlin's valuable paper, but also it served as an example 

 of the occasional utility of a device for the enhancement of 

 resolution by the introduction of an analysing Nicol prism into 

 the path of the rays. He found that a very small illuminant 

 was also an advantage. 



The President said they were all much indebted to Mr. Ainslie 

 for his remarks on the paper, and the thanks of the meeting 

 were voted to the author. 



The President invited Vice-President David Bryce to take 



