ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 243 



size. Now dip it in a solution of gelatin, draining off the superfluous 

 liquid, and allowing it to dry. The squares can then be scratched on 

 the film side with a pin. In whichever way the glass is prepared, it 

 must have the squares numbered consecutively in small figures. When 

 this glass is inserted in the eye-piece, each square covers a small portion 

 of the field, and the squares being numbered, the location of any par- 

 ticular object can easily be signified to any number of observers. 1 ' 



(4) Photomicrography. 



Scheffer's Microscopical Researches on Plate-grains. — W. Scbeffer 

 has devoted much attention to the above subject, and his results are 

 herewith summarised under the titles of his respective articles. 



Microscopical Researches on the Effect of the Persulphate and Ferri- 

 cyanide Reducers, as also on the Re-developing of Bleached Negatives with 

 Alcoholic Developers* — The author's object was to investigate the reason 

 for the difference in action of Lumiere's ammonium persulphate reducer 

 (soft result) and Farmer's ferricyanide of potash reducer (harsh result). 

 Suitable preparations were made, and the gelatin films sectionised by 

 the microtome, and examined microscopically. It was found that the 

 effect of the ferricyanide was limited to the upper part of the surface, 

 all grains then being dissolved, while in the lower part they were not 

 touched. The persulphate, on the other hand, penetrated the whole 

 film, and thus reduced all grains in an equal proportion. The author 

 quotes Werkner's redevelopment formula, which is especially suited for 

 changing harsh negatives into soft ones without loss of image in the 

 transparent part. 



Note on the Reversed of Solarised Negatives with Farmer's Reducer. ,| 

 If a bromide negative is exposed under a photometer in such a way 

 that the more transparent area of the field appears already as a positive 

 by solarisation, and the negative obtained by this is reduced afterwards 

 with Farmer's reducer, then a part of the reversed (by solarisation) 

 regions is changed again into a negative. This is best to be seen in 

 those places where the solarisation has not gone too far. Microscopic 

 examination showed that in the solarised parts the grains were equal in 

 size and evenly distributed over the whole thickness of the film. In 

 the less exposed parts the size and quantity of grains in the upper parts 

 were both greater. Under certain circumstances, reduction with ferri- 

 cyanide of potassium would invert the relative portions of transparency 

 of these two parts, e.g. if the reducer had penetrated down to the half 

 of the two films equally. In one case the greater quantity of the grains 

 would have dissolved, and only a very slight opacity remain ; in the 

 other, comparatively more of the grain would remain unattacked, and 

 consequently the parts, formerly more opaque, would be relatively more 

 transparent after reduction. 



Microscopical Researches* on 'the Size and Distribution of the Plate- 

 grains. % — The author illustrates his researches by a series of nineteen 



* British Journ. Photog.. liii. (1900) pp. 964-5 (9 tigs.). 



+ Tom. cit.. p. 1027 (2 figs.). 



i Op. cit., liv. (1907) pp. 116-20 (19 figs.). 



