Knowledofe. 



With uhicli is incorporated Hardwicl<c's Science Gossip, and the Illustrated Scientific News. 



A Monthly Record of Science. 



Conducted by Wilfred Mark Webb. F.L.S., and E. S. Grew, M..\. 



AUGUST, 19 11. 



THE TRUE STRUCTURE OE THE DL\TOM 



VAE\'E. 



B 



T. F. S.MITH. 



It is not within the scope of this article to enter 

 into the biological details of Diatoms, the writer 

 assuming, and rightlv too, he thinks, that thev are 

 already known to the readers of " KNOWLEDGE." 



His purpose is rather to 



treat of them as test 

 objects: of the part they 

 have already played in the 

 development of the achro- 

 matic microscope : of the 

 part thev still should pla\" 

 r '^\SSm'\ '" opening up the road 



r ■ i ""iv, \ towards further advance- 



'<■ ': ; " 1 ment. He also wishes to 



state his own \iews of 

 the structure of the valve, 

 the nature of which has 

 given rise to so man\- 

 discordant opinions. 



In Ehrenberg's monu- 

 mental work, " Die Infu- 

 sionsthierchen," published 

 in 1Sj8, diatoms are treated 

 as animals, in opposition to 

 almost all the authorities 

 of that day. This can be 

 of but little importance to 

 us now. 

 how a 



led astra\- b\- the follow- 

 ing up of preconceived 

 opinions. Of greater in- 

 terest, so far as diatoms are 

 concerned, are the superb 

 coloured drawintrs sriving 



save in showing 

 great mind can be 



us a clear insight of the definition of the microscope 

 then. He describes the various species, some as 

 smooth, some as striated : yet, on carefullv going 

 over the corresponding striated forms, under the 

 microscope, it will be 

 found that the\' can all lie 

 resolved b\- an inch objec- 

 tive of -30 X..\. 



\\'hile upon the subject 

 of comparative capacity, 

 it may be of interest also 

 to give an example from 

 a slightly later date. 

 One species described and 

 figured b\' Ehrenberg as 

 smooth is Xai-iciihr (now 

 Pleiirosii^iini i liippdCiunpiis. 

 In 1.S41. a Mr. Harrison. 

 of Hull, discovered longitu- 

 dinal striae upon this form, 

 and sent specimens to the 

 -Microscopical Society of 

 London, for confirmation. 

 .\fter keeping them for 

 si.x months the\' were re- 

 turned to him w ith the inti- 

 mation that the members 

 could see nothing. A little 

 later he also discovered 

 transverse striae on the same 

 diatom valve, but found 

 them more difficult to ex- 

 hibit. These can be easih' 

 resolved now b\- a half- 

 inch objective of -50 X..\. 



Figure 2. 

 U nder layer. 



Figure 1. 

 Upper layer. 



Two layers of a valve of Plciirosigiiia lUigiiltjtKiii. In each case entirely separated from the other and left sticking to the 



Taken with a six inilliinetres objective, by Zeiss, -95 N..'\.,and enlarged twice. 



cover glass and to the slip respectively 



2S9 



