530 



to investigate, and had received from him a number of results^ 

 some of which were quoted, which gave a practical resolving limit 

 of nearly 80,000 lines to the inch, multiplied by the numerical 

 aperture of the objective, results which were far higher than 

 those predicted for medium and high apertures by Mr. Conrady. 

 These resolutions were not blue ones, but white, and many of 

 them had been checked by himself. He thought, therefore, that 

 they would be justified in rejecting this particular application of 

 the Abbe theory to dark-ground images. 



Mr. Conrady, replying, pointed out that refraction, etc., by 

 the object was not an alternative to, but was included in, the 

 Abbe theory, as the scientific treatment of optics dealt with 

 wave-motions only, and made no distinction between refraction, 

 reflection and diffraction, which were peculiar to the narrow 

 treatment in elementary textbooks. The experiments described 

 by his critic were far too crude to either confirm or disprove his 

 (Mr. Conrady's) results, as white light had been used, and on 

 miscellaneous objects. It must be remembered that white light 

 included wave-lengths and resolving powers of from three- 

 quarters up to one and a half times that usually adopted as an 

 average (that is, light of wave-length 5,500 A.U.). 



Mr. A. A. C. Eliot Merlin, F.R.M.S., sent a "Note on a 

 Photograph of the Secondary Structure of Naviculci Smithiiy 

 This had reference to some observations communicated to the 

 Club, October 18th, 1907, and, the writer said, left no room for 

 reasonable doubt of the objective reality of the structure de- 

 scribed. The photograph was obtained at a direct magnification 

 of 2,900 diameters, with an apochromat J in. of r42 N.A., 

 and an axial illuminating cone of 0*5 NA. This exceedingly 

 undesirable procedure was rendered necessary in consequence 

 of the exterior surface of the valve being markedly convex, so 

 that with a large cone it was found practically impossible to 

 focus accurately at the same time more than one or two of the 

 primaries on the camera screen. 



Mr. A, Morley Jones said that he had recently mentioned to 

 Dr. Butcher, of Blackpool, that this paper by Mr. Merlin was 

 to be read, as he thought that as Dr. Butcher had been studying 

 this diatom for some years he might like to send a few photo- 

 graphs for comparison with Mr. Merlin's. In reply Dr. Butcher 

 sent some notes on the subject (then read), and a series of ten 



