rrK'riii-:K kM':m.\km<s c^x tup: true STRUcrrK'R 



Ol- Till': DIATOM \AL\'|{. 



l\\ 



T. |-. SMITH 



A (11 ANCI-: discovery, often the only one fortunate 

 chance of a hfetime, has enabled the writer to |)usli 

 our detinite knowledge of diatom 

 structure still some steps onward 

 from the point reached in his 

 last article. Trustinj^ that the 

 results here shown will prove 

 of both interest and value, he 

 now begs to lay them before 

 the readers of " KNOWlJiDGl",." 

 He wishes also that this article 

 should serve a twofold purpose ; 

 the second as illustrating what 

 can be done by high-power work 

 in photomicrography w ith cheap 

 lenses. The illustrations for the 

 last article were all produced 

 by expensive apochromatic ob- 

 jectives. I'or the present one, 

 only the ordinary achromatics 

 were used. Most of the nega- 

 tives reproduced here were taken 

 with his own lenses — a dry one- 

 sixth inch of 0-85 N.A. (onlv 

 one with this), and an oil- 

 immersion one-twelfth-incli of 

 1-30 N.A., both by Swift cS: Son. 

 For the others he is indebted 

 to the firm of Mr. Charles Baker, 

 of Holborn, who kindly placed 

 one of their one-twelfth incli 

 oil-immersions of the same iipvr- 

 ture at his disposal. The figures 

 may be allowed to speak for 

 themselves, though it ma\- be 

 pointed out that the apochro- 

 matic used on the previous 

 occasion had an aperture of 

 1-40 N.A. and was besides the 

 finest of its class; while the 

 achromatics used to illustrate 

 the present article were limited 

 each to 1 • JO. To say nothing 

 of apochromatism, this difference 

 upon many objects may not 

 mean much. When, however, 

 optically separating closely con- 

 nected structure the extra ten 

 points tell. 



One thinks it necessary for 

 the benefit of others to call 

 attention to these matters. It 

 has been, indeed, the habit of 

 some writers to throw cold 

 water upon the employment of 

 the ordinary lenses for photo- 

 graphy in high power work. In 



the language of 

 powers, when 



FiGUKE 3SJ. 



Plcurosigma formosuin, showing torn 

 fibrils projecting over the empty space where 

 I)art of the under layer has sunU to the 

 slip. Magnified one thousand six hundred 

 diameters. 



K.l'Ul-; J84. 

 The under layer of the same valve, show- 

 ing where the sunken part has broken oil. 



one : '" That while with low- 

 ed in conjunction with suitable 

 screens, excellent pictures can 

 be produced . . . with high 

 powers this is not the case, 

 and those who desire to obtain 

 the very finest results possible 

 by the aid of photogra|)hv, 

 should certainly obtain the ser- 

 vices of the apochromat rather 

 than of its cheaper rival."' 

 Indeed, in an annual of micro- 

 scopy, the same writer has 

 produced two plates, to show- 

 by contrast the difference of 

 appearance between the same 

 object, when photographed bv 

 the apochromatic and the ordin- 

 ary achromatic objectives. Yet 

 he admits in his last book that 

 it is often difficult to tell at 

 first the difference visually be- 

 tween the two, as the present 

 cheap lenses are made. 



Now this present writer will 

 admit that Jifter reading the 

 words, " the very finest results 

 possible," it was with fear and 

 trembling he approached the 

 task of high power work with 

 the cheaper rival. Vet after all, 

 there is no magic required in 

 photomicrography. He has 

 always maintained that the 

 common objective will produce 

 as good an image in a photo- 

 graph as it does visually. The 

 image upon the screen, whether 

 good or bad. is really the thing. 

 .After, it is the same process as 

 in ordinary pln)tography. Of 

 course, one is aware that in 

 many lenses there is a want 

 of coincidence between the 

 chemical and the visual foci. 

 The use of orthochromatic 

 plates in nearly every instance 

 will correct this discrepanc\-, 

 and the same kind of plate, 

 usici with a suitable screen, 

 will correct the remainder. .\n 

 ordinary achromatic with a red 

 correction will work upon an\- 

 kind of plate. 



.\n eminent authority in 

 microscopy confirms this opin- 

 ion. .Mr. E. .M. Nelson, in one 



350 



