138 PHOTO-MICROGEAPHY 



to the amount of oblique light ; and if a faint negative be obtained it is better at 

 once to give three times the exposure to the next one. It may be better to use a 

 monochromatic screen or glass as used in our case when taking the photograph 

 shown in Fig. 6, Plate IV. ; but this varies very much with the ability of the 

 operator, the amount of oblique light used, and the beauty of the specimen, for 

 some are so much better marked than others. It is not at all improbable the 

 photographer may here require to use a i"35 condenser because it admits so much 

 more light at any given obliquity. We can strongly recommend the new one by 

 Conrady, or the well-known N. A. 1-40 by Powell & Lealand, Fig. 53, p. 100. 



Seeing we may be now using over 1000 diameters, according to the eye-piece used 

 and the camera length, the greatest attention must be exercised in all adjustments, 

 and the dark slide must be put into the camera in the most gentle manner possible, 

 whilst opening the draw-slide cannot be effected with too much care. Centering the 

 condensers and the light become now more than ever acts of refinement, and all 

 movements around or about the table whilst taking the photograph must be strictly 

 forbidden, even though it is supported on cement feet apart from the floor. 



The Pleurosigma angulatum, Figs. 5 and 10, Plate IV., is another test object 

 and one of classical interest. It can be procured, mounted on the cover-glass, which 

 means in air, or in a highly refractive medium such as realgar. It has tiuo principal 

 planes of focus, and much difference of opinion exists as to which is the correct one. 

 For some reason, which is not very easy to discover, many microscopists find the 

 diatom easier to photograph if mounted in air ; and the easier plane of focus, that one 

 adopted by Dr. Zeiss in his celebrated picture, where black hexagonal marking- 

 surrounds a white centre. But other authorities are equally strong in their opinion 

 that the true focus is where the hexagonal markings are white and the centre portions 

 dark. The last pictures taken by Dr. Van Heurck with the new Zeiss N. A. i '6 

 objective and the attendant paraphernalia seem to show that after all the " black dot" 

 may be more correct than the white one. As before stated, the white one is the 

 easier to photograph, for the dark dot seems never to be sufficiently defined to look as 

 sharp as we should like it. It is also more often ovate, and this peculiarity we have 

 often noticed in photographs taken by others more experienced than ourselves. No 

 arrangement that we are aware of will cure this peculiar defect, but it is easy to 

 overlook unless special attention be paid to the point. 



When specimens are mounted in realgar another appearance may often be seen. 

 It has been called the " pearl dot " where each white dot — which is now seen dusky 

 around its base — seems literally elevated from its surroundings, whilst it is crowned 



