ZOOLOGY AND BOTANY, MICROSCOPY, ETC. . 647 



immersed iu a drop of oil of suitable refractive index placed between 

 the objective and condenser. 



The radial dovetails, whose arcs are both struck from a common centre 

 — the point of the steel pin — are built solid, brass bearing on gun- 

 metal. They are operated by specially arranged steel tangent screws of 

 0"5 mm. pitch. All movements can be clamped after adjustment. The 

 large circle is divided to ' 5° reading to 1 in. by a vernier. 



Watson's New Holos Immersion Paraboloid. --This apparatus 

 (fig. Ill) gives an intensely black background on a brilliantly illuminated 

 object with high power objectives up to 0*95 N.A. 

 The maximum effect is obtained by using a brilliant 

 light and a bull's eye. It is specially suited for 

 the exhibition of unstained living; bacteria. 



(4) Photomicrography. 



Photography by Reflection under Contact.* FlG 11L 



E. E. Fournier d'Albe, under the above title, de- 

 scribes a new process likely to be of interest to photographers of all 

 kinds. In the usual methods of contact photography a copy is taken of 

 the original or negative by allowing light to pass through the latter 

 on to a sensitive surface : the resulting picture is due to differences 

 in opacity in the various points of the original or negative. But the 

 author's new process consists in transmitting the light in the reverse 

 direction, and in producing a picture, not by differences of opacity, but 

 by differences of reflecting power in the original. The obvious objec- 

 tion to such a method is that the sensitive film, being exposed to a 

 uniform incident illumination coming through the back of the plate, 

 will be uniformly fogged, and the resulting positive will be marred 

 by a brightness which invades and obliterates all the dark portions. If 

 this difficulty can be overcome, we obtain a method of copying any flat 

 picture or design without a camera ; and we avoid the difficulties of dis- 

 tortion, curvature of field, chromatic and spherical aberration, and so 

 on. When the original to be copied has no half-tones, it is possible, 

 by suitable exposure and development, to eliminate the fog entirely. 

 The general principle is to employ exposures and developers which, in 

 ordinary photography, " suppress the detail in the shadows," or, in other 

 words, confine the developed image to those portions which have re- 

 ceived the maximum illumination. The author gives details and illus- 

 trations of the method. 



But the fog may also be eliminated by reduction and subsequent 

 intensification. Howard Farmer's reducer (potassium ferricyanide and 

 hyposulphite of soda) dissolves away the fog more than the full tone if 

 sufficiently concentrated. The negative is intensified with mercuric 

 chloride and silver-nitrate. The best results are obtained with slow 

 plates of the " photomechanical " class, and the developer used was the 

 following : — Solution No. 1 : Hydroquinone 80 gr., pot. metabisulphite 

 120 gr., pot. bromide 10 gr., water, etc., 10 oz. Solution No. 2 : Caustic 

 potash 200 gr., water, etc., 10 oz. Equal parts of both solutions are mixed. 

 The best fixing agent is potassium cyanide, oil account of its solving 



* Sci. Proc. Eoy. Dublin Soc, sii. (1909) pp. 97-100 (1 pi.). 



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