577 



NOTES ON DIATOM STRUCTURE. 



By A. A. C. Eltot Merlin, F.R.M.S. 



{Read April 21th, 1915.) 



I venture to bring to your notice a very beautiful form of 

 tertiary structure which I have recently found on a variety of 

 Aulacodiscus Comberi from Oamaru. The valve is on a styrax 

 type-slide of 230 forms from that locality and is covered with 

 a network of dark, well-defined secondaries, except on the parts 

 occupied by the large primaries. Each of the dark secondaries 

 has been found to be split up into three or four parts by a bright 

 cross-bar arrangement. This structure requires a good oil- 

 immersion objective and a very considerable magnification to 

 render it readily discernible, but it is in no way a glimpse object, 

 and when well seen reminds one of the bridges of bright matter 

 that are frequently observable crossing the umbrae of sunspots. 

 Photograph No. 1 exhibits clear indications of the structure 

 in question, x 2,150 diameters, although it cannot be photo- 

 graphed as plainly as it can be demonstrated visually. Inci- 

 dentally the photograph serves to prove the fact that with a 

 power of 2,150 diameters there is no excess of " empty magnifica- 

 tion," when employing a good lens of 1*4 N.A., for few will 

 comfortably see the structure therein without the aid of a low- 

 power magnifier.* 



Two other photographs are sent herewith for your inspection. 

 These were secured under the following circumstances. Mr. 

 Nelson wrote to me that he had discovered that Coscinodiscus 

 Simbirskii, which with ordinary transmitted light resembles Cos- 

 cinodiscus asteromphalus, looks like Actinoptychus splendens when 



* The photographs referred to in this paper contain details of such 

 a nature that only a drawing could adequately represent them for 

 purposes of reproduction. 



Journ. Q. M. C, Series II. No. 77. 32 



