E. M. NELSON ON MINUTE DIA.TOM STRUCTURE. 431 



cone the structure was rendered sufficiently visible to enable us 

 to hold it steadily. 



Oblique illumination completely obliterates this structure, which 

 can only be seen by means of a direct axial cone of maximum dry 

 aperture. 



Addendum. 



{Read May 2Ut, 1897.) 



This note is merely a postscript to my last communication, to 

 record the further discovery of some minute diatom structures. 

 Campylodiscus omatus, this very uneven diatom, has an ornamental 

 border round it, on the exterior edge of which is a row of small 

 compartments, and on the interior edge a row of much smaller 

 ones. These large exterior compartments have a delicately 

 perforated membrane over them, and so too have the interior 

 ones, but the delicate structure on the interior ones is coarser than 

 that on the exterior. It is just what might have been expected 

 as it is in perfect conformity to the law of diatom structure 

 — viz., " coarse in the centre, fine at the periphery." On one 

 occasion I had placed this slide on my student's stand, and was 

 examining it roughly, the objective being an old student's |-in., 

 and the illumination daylight, when to my astonishment I saw 

 what appeared to be the resolution of the fine radial striae in the 

 peripheral compartments. A moment's reflection convinced me 

 that it was nothing but a ghost, the real structure being far- 

 and-away beyond the grip of that or any other dry lens ; more- 

 over, the coarser structure in the interior compartments was not 

 resolved ! This is an interesting instance of the ease by which 

 a false ghost can be produced by rough-and-ready microscopy. 

 Some microscopists are of opinion that false ghosts are entirely 

 monopolised by critical workers, who use elaborate condensers and 

 wide-apertured objectives, etc.; and think that the elementary 

 student's microscope suitable for biological or histological work is 

 free from all these objections ; but such evidently is not the case. 

 Actinocyclus Ralfsii. — This diatom has a finely perforated 

 membrane extending all over it. This membrane is probably 

 similar to that in Eupocliscus Argus, only much finer. Inside 

 the well-known white dots, elaborate ghost patterns can be made ; 

 it is, however, not difficult to discriminate between the ghost and 



