421 



X 4000 direct. He then thought that practical finality on this 

 subject had been reached. A few months ago, however, on 

 returning to London after an absence of some five years, the 

 assistant secretary of the Royal Microscopical Society had 

 drawn his attention to the paper on the Podura scale by Mr. 

 Nelson in the Journal R.M.S. for 1907. In this paper is 

 described and figured a fine mycelioid structure, and in addition 

 " some minute horizontal filaments of great tenuity joining the 

 vertical lines, the visibility of which by transmitted light re- 

 quires a fine objective and skilful manipulation." Mr. O'Donohoe 

 said he was at once seized with a strong desire to see, and if 

 possible photograph, this structure. With a condenser cone of 

 N.A. 0*65 he produced a " black-dot " image (photograph shown). 

 This exhibited the fine transverse filaments, which were identical 

 with those shown in the following "white-dot" image, taken 

 with a cone of N.A. 0*85. Mr. Nelson's criticism of the " black- 

 dot " photograph was that it showed the heads of the "exclamation 

 mark " convex and not concave, and that the fine markings 

 between the " exclamation marks " were rather like diatomic per- 

 forated structure instead of like fine bars. Mr. Nelson suggested 

 that the image sought for was at a trifle higher focus, and is 

 seen in a blaze of light, with a full cone of N.A. 1*0 from the 

 condenser. The author then used a Watson immersion con- 

 denser of N.A. 1-30 cut down to 1*0, and the image sought was 

 at once observed ; the fine structure and concave heads were 

 well seen. Efforts to photograph these with aperture N.A. 1*0 

 proving unsuccessful, a cone of 0*85 was used. This gave the 

 fine lines, but not the concave heads. Further attempts, how- 

 ever, with full cone (N.A. l'O) at length proved successful, and 

 of the photograph shown Mr. Nelson had written that it w r as 

 the best photograph of the Podura scale he had seen, and hoped 

 that it would be accepted as the true view of the scale. Mr. 

 O'Donohoe also showed a photomicrograph of Mr. Nelson's 

 "electric-light" image, described Journal JR. M.S., 1907, p. 399. 



Dr. Spitta could not agree with Mr. O'Donohoe as to the cause 

 of the markings observed. He thought they were probably due 

 to the cusps of the next lower layer. While congratulating the 



