24-4 SUMMARY OF CURRENT RESEARCHES RELATING TO 



photographs, showing various stages and results in the development of 

 a plate The first stage of development always seemed to originate in 

 the protrusion of small rod-like processes, usually knob-terminated, from 

 the grains. These processes, or filaments, seemed to be more numerous 

 on the smaller grains than on the large ones. The impression suggested 

 to the observer was that the results were in the nature of an explosion, 

 which took place during the exposure, small bodies being apparently 

 shot away from the grains and making their way through the gelatin 

 either in straight or in irregularly curved lines. Both the terminal 

 knobs and the filaments acted as germs, at which development com- 

 mences. Sometimes the filament is hardly visible even with the highest 

 power oil-immersions. Thus the germs at which the formation of the 

 developed grain commences are situated outside the original grains, and 

 also the further stages of development are outside the original grains. 

 The developed grains are always clumsily-aggregated masses. 



It would seem that in an exposed film the grains may be divided 

 into three classes : — (1) Original grains, i.e. grains which have germs 

 round themselves, which germs are the points where development 

 commences. These original grains are not dissolved by development. 

 (2) Dissolving grains — grains which show no germs, and which are 

 dissolved either partly or entirely by chemical development. (3) 

 Developed black grains. 



Jficroscopic Researches on the Plate-grain* — In this article the 

 author examines the relations of " dissolving " and " original " grains 

 under different conditions of development and exposure. He infers 

 that the solubility of the dissolving grains in chemical developers is 

 governed by the exposure, and that the solubility increases at the com- 

 mencement corresponding with the exposure up to a maximum, after 

 which it decreases with the increasing exposure. He also found that 

 the solubility of the dissolving grains, as* well as the size of the developed 

 grains, corresponded with the concentration of the developing solution. 

 The size of the developed grains also depended on the number of grains 

 in unit volume of the gelatin. 



& v 



Mees, C. E. K. — Screen-plate Colour Photography. 



[The author describes some twelve processes, and discusses the scientific 

 principles which underlie them.] 



Journ. Soc. Arts, lvi. (1908) No. 2878, pp. 195-204 (6 figs.). 



(5) Microscopical Optics and Manipulation. 



Correction of the Astigmatism of Doubly Refracting Prisms.t — 

 C. Tissot and F. Pellin refer to the deformation of image produced in 

 various degrees by all doubly refracting prisms. In the case of a nicol, 

 it is only the extraordinary rays which contribute to the image, i.e. rays 

 which do not, in general, remain in the plain of incidence. The result is 

 a dyssymmetry which can be proved by an easily shown astigmatism. 

 Thus, if a homocentric beam, limited by a narrow circular diaphragm, be 



* British Journ. Photog., liv. (1907) pp. 271-3 (7 figs.). 

 t Comptes Rendus, cxlv. (1907) pp. 866-7 (3 figs.). 



