NOTE ON THE PHOTOGRAPHIC TECHNIQUE. 



The installation used in producing the photomicrographs of Toxopneustcs was that manufactured by Zeiss of 

 Jena in possession of the College of Physicians and Surgeons. In order to have as few variants as possible enter 

 into the work, the following method of procedure and arrangement of apparatus was adopted and adhered to 

 throughout. The adjustment of focus was left entirely with Professor Wilson as being most familiar with the special 

 points desired; the exposure was then made so as to slightly overtime the plate, and it was subsequently intensified; 

 where advisable Strong's adjustable false stage was used in order to bring into the same focal plane a second or 

 third point of interest, and it was found that notwithstanding the short working distance of a 2 mm. lens the slide 

 could be considerably tilted. The optical combination was an Abbe substage condenser achromatic i N.A. — a 

 Zeiss 2 mm. oil immersion ajDochromat and projection ocular No. 4. The camera length was so adjusted that the 

 image represented a magnification of the object of 950 to 1000 diameters. The illuminant employed was the 

 electric arc, which was modified in the following manner. The condenser and objective were so focussed, that the 

 image of the crater was projected with the image of the object on the focussing screen ; a finely ground glass 

 screen was then interposed in the path of the light rays near the condenser; this not only gave an evenly 

 lighted field, but prevented the formation of rings or lines by diffraction. The stain used by Professor Wilson being 

 the iron alum and ha-matoxylin stain of Heidenhain, the objects had a light blue tint by transmitted light; there- 

 fore isochromatic plates were used with a colour screen made by dyeing a lantern-slide plate from which the silver 

 salts had been removed with an alcoholic solution of tropa?olin, a cover glass being cemented to the plate by 

 Canada balsam which rendered the dyed gelatine quite transparent. A single solution hydrokinone developer, 

 of a constant composition, was used, the exposure varying from three minutes with a fresh developer, gradually 

 increasing to ten minutes as the developer became more oxidized. I cannot help alluding here to the excellence 

 of the sections from which these photomicrographs were made, the perfection of which, in every part of the 

 technique of cutting, staining, and mounting, was such as to make working with them a pleasure. In no case 

 have the negatives been retouched or even spotted. 



EDWARD LEAMING. 

 May, 1895. 



