i70 



KXOWIJ-.nGR 



Octoukk. 1912. 



whicli almost fvtTV diatom fnistulf is beset has 

 been one of the most interesting enquiries of the 

 students of these forms since tlie introdnrtioii nf 

 tile homogeneous, and especially 

 the apochromatic, objectives : 

 and it cannot be doubted that 

 certain jjeculiarities of struc- 

 turi' have been demonstrated 

 which were never before seen." 

 One is aware that this does 

 not constitute the whole work 

 of the microscope, or even the 

 better part, yet it possesses 

 this distinction that it has 

 paved the way towards niakiiij,' 

 other work possible. The 

 President of the Royal Micro- 

 scopical Society said on the 

 reading of a recent paper on 

 diatom structure, that : " The 

 Society was always grateful 

 for an\' work on diatoms, as 

 this work had, more than any 

 other, led to the perfecting of 

 our microscopic lenses." 



Recurring to Messrs. Nelson 

 and Karop's paper : occupying 

 the centre of their plate is 

 Cosciiiodiscus usterompJialiis. 

 of w hich it may certainly be said the finer structure 

 had never been seen nor figured before. More. 

 indeed, than T. faviis the 

 visibility of the finer details 

 u|)on this form is due entirei\- 

 to the oil-immersion objectixf, 

 being beyond the compass n( 

 a dry lens. The general form 

 and appearance of this diatom 

 under an apochromatic twenty- 

 four millimetre of Zeiss is 

 given in Plate 1 of tlie 

 Dallinger "Carpenter," and in 

 the same plate a photomicro- 

 graph of a portion of the \al\L- 

 at two thousand diameters. 

 Both are by Mr. E. M. Nelson. 

 It may seem rash of the 

 present writer to attempt to 

 follow in the footsteps of so 

 splendid a manipulator. Per- 

 haps he would not, had it not 

 been to prove a point — the 

 identity of structure in all 

 diatotns. This form also is 

 well to the front, much dis- 

 cussed, just now, when it is 

 as well to be in the fashion. 



It will be seen that the rendering here differs 

 from Mr. Nelson's, though it is in accordance with 

 Dr. T. W. Butcher's, in his paper published in 7V(c- 

 Joiirihil of the Royal Microscopical Society inv 

 December last, Mr. Nelson always went for tlie 



Figure 405. Coscinndisciis ttstcroiuplialus. 

 centre, but from ;inolher valve. Objective 

 used, Baker's oil iiniuersion, one-twelfth inch 

 of 1-30 N..\., ordinary plate, no screen, mag- 

 nification two thousand seven hundred and fifty 

 diameters, exposure twelve minutes. 



^M mm 



I'h.rni liiii. Ci>si-ui(Hltsi.iis iistcroiuplt, litis. 

 I'loiu llu: side of the same \alve as I'igure 404. 

 Objective. Baker's one-twelfth inch oil-immer- 

 sion, ordinary plate, pot green screen, magnifica- 

 tion two thousand seven hundred and fifty 

 diameters, exposure three hours. 



black dot as being the most truthful ; the present 



writer prefers the white as being the most pretty. 



,\s for truth, he does not believe that there is a 



pin's difference to choose 



between the two. 



Into the merits of the con- 

 troversy between these author- 

 ities it is not proposed to 

 <nter here. The writer's own 

 ' xperience has been unfor- 

 tunate in this matter. Wishing 

 to find out for himself, and 

 seeing a slide advertised as 

 being prepared to show Mr. 

 Nelson's tertiar\' structure, he 

 Nought one, only to be con- 

 fronted with a vision of eye- 

 >pots. They were upon forms 

 -() small, too, that it would 

 liave been difficult to make 

 lut the secondary structure 

 ( ven had they been the right 

 way up. His own knowledge, 

 therefore, upon this point still 

 remains in abeyance. 



Figure 404, showing the 

 whole disc, is taken by a one 

 inch of Swift & Son, of 0-30 

 N..\., magnified three hundred 

 nd fifty diameters. Mr. Nelson's is under dark 

 round ilhiminatioii. so in this case there are no 

 points of coini>arison. I'igures 

 405 and 406 show the normal 

 aspect with white dot focus 

 of parts of the valve under 

 an oil - immersion, rnagnified 

 iwii ttiousand seven hundred 

 ,111(1 tiftx" diameters. Dr. 

 IWitclur has so accustomed 

 us to heroic magnifications 

 that one is bound to follow 

 his example, if only to compare 

 11 suits. Vet one remembers 

 the time when one thousand 

 diameters was looked upon as 

 the efficient ma.ximum. Yet 

 the lenses were the same as 

 now : further, nothing so far 

 has been seen under the 

 microscope which cannot be 

 reailiK made out with this 

 |)(>\\tr. The white dot in 

 l''igure 405 must be taken 

 with a qualification. It is 

 really half w bite and half black 

 dot, due to the whole of the 

 field not being in quite the 

 same plane, (^iie other point besides it does show, 

 however : how beautifully the objective was in 

 adjustment under the tube-length employed. 



The next two Figures. 407 and 40S. iinsent 

 examples of torn structure in this diatom. We have 



