Royal Microscopical Society. 159 



Dr. Pigott are a spurious appearance, come into view. That this 

 train of phenomena is reversed in certain scales would appear to 

 show a difference between the upper and the lower series of ribs. 



I send the following photographs in illustration of the above 

 remarks : — 



No. III. Photograph of Begeeria Nigro-maculata ; 3100 

 diameters. — The scale was '0035 inch long and '0018 broad. 



No. IV. Photograph of the same scale ; 2300 diameters. 



No. V". Photograph of another scale of Degeeria Nigro- 

 maculata ; 2700 diameters. — The scale was ■ 0031: inch long and 

 •0013 broad. 



No. VI. Photograph of the same ; 2300 diameters. 



No. VII. Photograph of the same; 960 diameters. 



No. VIII. Photograph of the same scale, with several others 

 in view ; 960 diameters. 



No. IX. Photograph of part of the large scale of Lepidocyrtus 

 curvicollis on the slide sent by Dr. Pigott; 960 diameters. 



No. X. Photograph of Lepidocyrtus scale o-g-o^h of an inch long. 



To the foregoing photographs of Degeeria and Lepidocyrtus I 

 add a few of certain diatoms. Long since Mr. Wenham arrived at 

 the conclusion that the markings on diatoms were in fact due to the 

 presence of hemispherical elevations or bosses. Fresh attention was 

 drawn to this view by the paper of the Eev. J. B. Eeade " On the 

 Diatom Prism and the true form of Diatom Markings."* Mr. Keade 

 appears to have attributed to his Diatom prism (which I have care- 

 fully tried), just as Dr. Pigott has to his " Aplanatic Searcher," the 

 superior definition, which is, in reality, ascribable to the improved 

 quality of the objectives, and especially of the immersion objectives 

 recently produced by certain makers. In point of fact, with modern 

 immersion objectives, and especially with the immersion |th and iV^h 

 of Powell and Lealand, I find myself able to produce better efiects 

 with the help of an achromatic condenser, properly managed, than 

 by the Diatom prism. 



More important even than the paper of Mr. Eeade, however, 

 were the experiments of Schultze on artificial diatoms, to which 

 Mr. H. J. Slack has recently drawn attention in his paper " On the 

 Patterns of Artificial Diatoms." t I have never experimented with 

 these, but have in my possession a number of photographs of them, 

 presented by my friend Dr. Maddox, which are of the most in- 

 structive character. That this \iew of the structure of diatoms is 

 substantially correct I do not now doubt. The details in the case of 

 each individual diatom will, however, require careful study. 



No. XI. Photograph of Pleurosigma Formosum; 4500 

 diameters. 



No. XII. Photograph of the same; 3100 diameters. — The 



* 'Monthly Microscopical Journal,' vol. ii., p. 4. f lb., vol. iv., p. 181. 



