443 



NOTES ON A PHOTOGRAPH OF THE SECONDARY 

 STRUCTURE OF NAVICULA SMITHIB. 



By a. a. C. Eliot Merlin, F.R.M.S. 



{Read March 2GtJi, 1912.) 



Plate 22, figs. 3 and 4. 



With reference to my communication to the Quekett Microscopi- 

 <jal Club, read October 18th, 1907, on a certain new secondary 

 structure of iV. Smithii, I now submit for your inspection a non- 

 critical small -cone photograph showing the structure in question 

 sufficiently plainly to leave no room for reasonable doubt of its 

 objective reality, should such yet exist in the minds of those 

 familiar with the perforated capped primaries of other diatomic 

 forms. The photograph (PI. 22, fig, 3) was obtained at a direct 

 magnification of 2,900 diameters with an apochromatic J in. of 

 1-42 N.A. and an axial illuminating cone of 0*5 N.A. This ex- 

 ceedingly undesirable procedure was rendered necessary in con- 

 sequence of the exterior surface of the valve being markedly 

 convex, so that with a large cone it was found practically im- 

 possible to focus accurately more than one or two of the primaries 

 on the camera screen ; it therefore appeared allowable under 

 such circumstances to bear in mind the adage that " half a loaf 

 is better than no bread," and to invoke the aid of the small 

 cone, which, you will see, has solved the difficulty by permitting 

 :a considerable area of the highly curved surface to be imaged 

 with sufficient general accuracy for illustrative purposes. In 

 some parts of the print the perforations are shown in white, and 

 in others in black-dot focus. It is unfortunatelv true that the 

 inherent defects of uncritical images are but too apparent to 

 the trained microscopist, and thus the rendering of variations 

 of size and shape of the secondaries leaves much to be desired 

 when compared with the clearly shown visual critical picture, in 

 which also the accommodating power of the eye makes some 

 amends for the lack of the same quality in the objective. For 

 purposes of compaiison, a critical photograph of the Cosciuodiscns 

 aster omphali IS showing secondaries x 3,300 is annexed hereto 

 (PI. 22, fig. 4) this form being sufficiently flat for such 

 treatment. 



I would earnestly impress upon the younger and more inex- 

 perienced members of our Club that, as a rule, photographs 

 obtained under similar conditions to that of the N. Smithii are 

 to be avoided by all who aspire to sound and good work, however 

 fascinating their easy production may prove: for, given the 

 necessary instruments and lenses, anybody with an elementary 



