444 A. A. C. ELIOT MERLIN ON STRUCTURE OF 2<!AV1CULA SMITBII. 



knowledge of ordinary photography, plus a few weeks' micro- 

 scopical experience, can turn them out in quantities with ease 

 and certainty, to the admiration of the unwary, for to such the 

 picture of a diatom with the impressive contrast that reminds 

 one of a new chess-board appears all that can be desired as the 

 truthful representation of a highly translucent structure pierced 

 by minute perforations and viewed by transmitted light ! When 

 the full, or nearly full, aperture of an oil-immersion objective 

 is brought into play, and diffraction effects are reduced to a 

 minimum, then the real difficulties commence, and are so for- 

 midable that few care to face them, especially as the results are 

 only appreciated by the initiated. 



Moreover, in spite of all the advantages of the small cone for 

 those who like to photograph at ease, it is certain that pictures 

 so obtained of unknown or uncertain structures should be received 

 with the greatest caution ; for although diftraction mciT/ occasion- 

 ally be a good servant, it is more than likely to prove a bad 

 master. Doubtful structures should invariably be confirmed by 

 at least visual critical observation. Thus, in the case of X. Smithii, 

 under the best visual conditions it is clearly seen that the primaries 

 are pierced with holes of varying size and number,* and the non- 

 critical photograph renders these coarsely, but with sufficient 

 accuracy for record pui-poses, so that they may serve as a guide to 

 what should be looked for and studied under preferable condi- 

 tions. Were the valve as flat as that of the Coscinodiscus 

 asteromphalus there would be no excuse, save sheer incompetency, 

 for such a poor rendering. A great and very real danger with 

 uncritical images is that, while wholly false diffraction effects 

 may be mistaken for genuine detail, real structure of an unex- 

 pected or complicated character may be quite as probably over- 

 looked or doubted. It is significant that the N. SmitJdi secondaries 

 escaped detection for long, although lenses fully capable of 

 revealing them existed ; and although the form had attracted 

 considerable attention it is likely that the secondaries w^ere seen 

 in uncritical images many times without their true nature being 

 grasped, even though it might have been expected that a hint 

 would be sufficient to any one familiar with the perforated capping 

 of other diatoms. 



In submitting these remarks and photographs I beg they may 

 be taken as an addendum to my previous communication on 

 N. S'lnithii {Journ. Quek. Micr. Clifh^ Ser. 2, Vol. X. p. 247). 



* jV. Smithii valves examined van^ con.siderably in size (from 0*15 mm., 

 to 0-04 mm. in length), bnt all exhibit similar secondary structure. 



Journ. Qukttt Microscopical Club, Ser. 2, Vol. XL, i\'o. 70, Ap7-il 1912. 



