THE society's NOTE-BOOKS. 197 



Mr. Bartleet has misunderstood what I said about markings. 

 I did not say that Pleurosigma fonnosiim had markings uj) to 

 130,000, but that some had. I have, however, at Mr. Groves' 

 suggestion, altered my statement, making them to run up to 

 1 00, 000th of an inch, Amphipleura pellucida 93,000 ; Navicula 

 subiilissitna \\2ls 112,000. A. Clarke-Smith. 



Having lately spent a good deal of time upon diatoms, the 

 present box of slides presents to me great interest. To draw 

 diatoms would be labour of love and infinite patience. A much 

 more ready way is to photograph them, and no structures lend 

 themselves more readily to the process than diatoms. A photo- 

 graph of Coscinodiscjis shows the areolated appearance of this 

 diatom, as seen with the 34^-inch objective. The appearance thus 

 shown is called primary structure, and to those not accustomed to 

 examine diatoms it would seem unnecessary to examine such a 

 diatom with the higher power. Another photograph shows a 

 portion of the same diatom made under a i/i2th oil immersion 

 lens. The diatom frustule is slightly arched on the surface, so 

 there is only a portion of it can be brought into focus at one time 

 with such a high power. The centre or peripheral portions are 

 therefore out of focus, the median zone being more or less in 

 focus. In this portion other structures are shown. Around the 

 primary areolatures are seen secondary areolations, and in the 

 centre of the primary areolations the appearance of structure which 

 under the magnifying power (950 to 1000 diameters) cannot be 

 adequately defined. With the latest apochromatic objectives of 

 Zeiss, giving a magnifying power of 2000 to 3000 diameters, the 

 structure in the centre of the primary areolations is shown to be 

 an exceedingly fine cribriform plate. These secondary areolations 

 and cribriform plates are often spoken of as the " ultimate struc- 

 ture " of the diatom. But who shall say that this is ultimate 

 structure ? This can only be decided when lenses have been pro- 

 duced possessing over those which we have at present in our 

 possession, still wider angles and proportionate power. 



I enclose also a photo of a well-arranged group of diatoms * in 

 my possession, as seen by a dark-ground illumination. Besides 



*We regret that we cannot reproduce these photographs. 



