284 Transactions of the Society. 



published a paper condemning the wide-angled cone.* This photo- 

 graph, afterwards withdrawn, was taken with one of the new oil- 

 immersion apochromatic y 1 ^ and an electric arc lamp, and with all 

 the resources of the Zeiss factory at the operator's hands, yet it 

 showed not an iota of advance upon results obtained thirty years 

 previously with dry achromatic objectives.! In brief it was, as I 

 said at the time, a repetition of Keade's "plate of marbles."J 

 When a wide-angled axial cone is used the hemispherules or 

 marbles disappear and in their place is seen the true structure, 

 which is a piece of siliceous sieve or net. Under the large cone 

 so sensitive does the object-glass become that it is possible to cut 

 optical sections, and visually separate the upper net from the lower 

 net. Those who in this manner have studied this diatom are able 

 to tell you from a slight difference in appearance whether you are 

 looking at the outside or inside net. With the narrower cone 

 there is indeed " a plate of marbles," but no one would have the 

 slightest suspicion that there were two plates, each capable of 

 holding their own marbles ; and as for recognising any minute 

 difference between the two nets, it is quite impossible with a small 

 cone to see any net at all with any lens, however perfect. With a 

 small-angled cone the benefits both of apochromatism and of large 

 aperture are entirely thrown away, and a really fine objective is 

 degraded to one of common place. This example of P. angulahim 

 has been selected for illustration, as it was a well-known case, and 

 will be remembered by some. Mr. T. F. Smith, who was then 

 working on the Pleurosigma, exhibited before this Society some 

 very beautiful photomicrographs in illustration of his theories as 

 to the structure of this genus. His system of illumination differed 

 from mine, as he used a bullseye in conjunction with a dry 

 achromatic condenser, which gives what may be called annular 

 illumination, the back of the object glass having the appearance 

 shown in K (fig. 46), which is not a large solid axial cone such as 

 mine shown in B. His method, by whatever name it is known, was 

 the very antithesis of the Abbe small cone, and, like mine, was 

 adversely criticised at that time by those who upheld Professor 

 Abbe's views upon the subject. Of the photomicrographs which 

 have been produced, and of the objects which have been exhibited 

 under small cone illumination, none may be said to present 

 structures minute enough to be considered difficult microscopical 

 images. The work which would be suited to low-power lenses of 

 medium capacity under large cone illumination requires under 

 small cone illumination high-power lenses of the greatest capacity. 

 Nearly a quarter of a century has now elapsed, and we are still 



* See this Journal, 1889, p. 721. 



t See Hogg on the Microscope, 2nd ed., 1855, p. 152, fig. 99. 



\ Rev. J. B. Reade, F.R.S., President R.M.S., Monthly Micr. Journ., ii. (1869)- 

 p. 5. 



