^16 



for the mathematical analysis to determine that question is 

 beyond that which I have read ; but it would seem to be some 

 sort of roller. 



The residual aberration in this lens is so small that it may 

 -correctly be called " aplanatic," for it is too small to make any 

 practical difference. As it is a single lens with spherical surfaces, 

 it is a very simple form of a dark-ground illuminator. 



ON THE "PSEUDOPODIA" (DIATOMS). 



By E. M. Nelson, F.KM.S. 

 {Read May 2Hth, 1912.) 



A SLIDE of Mr. Siddall's diatoms, having the so-called pseudo- 

 podia, was kindly lent to me by Mr. Angus. When the 

 pseudopodia were examined under a Leitz apochromatic 1/1 2th, 

 structure could be plainly seen inside the filaments. In one 

 from a Co.scinodiscus there was something remarkably like a 



53 



Fig. 1. Filament from Coscinodiscus. 



spiral filament in a tube. It is, of course, well known that in 

 <}ertain positions minute dots appear like a spiral, and vice 

 versa ; but on one occasion, when a good straight piece was 

 well seen, any experienced microscopist would have said that 

 it was a spiral structure. On two filaments coming from a 





Fig. 2. Filament from Biddulphia. 



Biddulphia the structure was quite difterent, for it resembled 

 .a minutely jointed antenna. 



The first notice of these, so far as I know, is by W, W. Wood, 

 Monthly Microsc. Journ. vol. 14 (1875), p. 255, fig. unnumbered. 



