140 



PHOTO-MICKOGKAPHY 



of the note," requires more magnification and a little higher N. A. Indeed, to do so per- 

 fectly an eighth, with fairly high N. A. and a fairly high eye-piece, or much camera exten- 

 sion is required. We say a fairly high N. A. because we are aware that N. A. is not 

 of so much consequence with this specimen. It is the colour correction that is of so 

 much importance, and any failure in this respect will be readily apparent visually by 

 showing the " white interior of the note " coloured, and photographically by failing to 

 show it clear, clean, and well defined. Indeed, nothing but the very best of 

 apochromatics exhibit this centre as strictly and absolutely white. Concerning defi- 

 nition, the white centre should in the photograph distinctly show a well-marked 

 constriction around its neck at the broadest part, and the whiteness should be 

 prolonged, becoming narrower and narrower until almost a line for about two-thirds 

 of the entire length of the black note itself. A shadow effect (?) should also be seen 

 under each "note." What these markings are due to is a matter of great dispute, 

 some thinking them depressions, whilst others regard them as hair-like appendages. 



Amphipleura pellucida, Kiitz (Fig. 4, Plate IV.). — Perhaps the most difficult 

 specimen, both to obtain as well as to photograph, is an Amphipleura pellucida, Kiitz. 

 None that the writer has ever seen can compare with those prepared in realgar by 

 Dr. Van Heurck, excepting those mounted by Mr. Gatrell. 



Some considerable practice is necessary before the beginner can even see the lines, 

 much less photograph them. It may be of service to assist him if we mention the 

 following : — We have found that to see the lines no auxiliary condenser is absolutely 

 necessary ; we can do so perfectly, and without any difficulty measure them with the 

 N. A. I'o condenser, the F line screen, and oblique light, but we admit they are 

 better shown with the 1*35 Conrady condenser. To photograph them so that the 

 lines shall be p>ure and of about one-third the thickness of the white space between 

 any two, the auxiliary condenser, as well as the 1*35 substage condenser oiled to the 

 slip, must be employed. It is also to be recollected that the final perfection of the 

 image, as seen on the ground glass with the 18 or 27 eye-piece, should be attained 

 by fidgetting the auxiliary condenser from side to side and up and down, rather than 

 by using more and more oblique light. This point to bear in mind is of all import- 

 ance, as we have found by so doing the lines are finer and the exposure enormously 

 reduced. Without the additional condenser, and not using so wide angled or so 

 perfect a substage condenser, we failed, after even aii hour had been spent with the 

 plate exposed, to obtain a good negative, and this we traced to the fact that the 

 heat either caused the realgar to melt — if it may be so said— or it upset the focus- 

 sing of the microscope itself But after repeated trials we discovered the advantage 



