ON THE PROGRESS OF SCIENCE. XIII 



drawn off, and the picture served in a bath composed of five parts of 

 concentrated sulphurous acid diluted in 100 parts of "water, moving it 

 about, as before, at intervals. After this double process, the carbon 

 almost entirely disappears from the lights and clear spaces, while it 

 remains in quantities proportional to the greater or less intensity of 

 action of the light upon the other portions, and thus the proof finally 

 reproduces the positive, but not perfectly, since the lights and half 

 tints are not pure, and the blacks lack somewhat of brilliancy and 

 perfectness. But the process is simple and good ; it remains only to 

 perfect it. 



M. Pouncy, another competitor, operates a little differently, but 

 obtains results equally satisfactory. His process differs in applying 

 the carbon before exposure of the proof to light the sensitive coating 

 being formed at once, of bichromate' of potassa, gum arabic, and 

 finely divided carbon, exposed not under a positive, but under a 

 negative plate. On removal, the plate is placed in a bath of pure 

 water ; after five or six hours* immersion, he washes under a stream of 

 common water, and the carbon positive is obtained. In this process 

 the manipulation is a little easier and more simple. The use of a 

 negative authorized the expectation of a better result ; but the expo- 

 sure is longer than in the mode of Gamier and Salmon, whose use of 

 a positive avoids the chances of accident which attend the negative 

 plates in the hands of the operator. Messrs. Pouncy, Gamier, and 

 Salmon, share the prize with Mr. Poitevin, who has the merit of anti- 

 cipating these photographers, whose methods are only an advance on 

 the process which Mr. Poitevin published in 1856. 



In order to enable the public to derive full advantage from the 

 photographic negatives made officially for the British Government, 

 from rare and valuable objects in public and other collections, British 

 and foreign, the Committee of Council on Education, for Great 

 Britain, has caused an office for the sale of photographic impressions 

 from such negatives, to be established in London. Photographic 

 negatives made by order of the Trustees of the British Museum, and 

 for the War and other Government offices, will also be sold. The 

 tariff for unmounted impressions will be as follows : A single impres- 

 sion, the dimensions of which containless than 40 square inches, e.g., 

 5 X 7 inches, or 4 X 8 inches, --5d. Above 40 square inches, 2 id. 

 should be addetl to every 20 square inches or under. A detailed 

 list of the objects photographed is printed, price 2d. The depart- 

 ment does not charge itself with the mounting of impressions, as the 

 public is able to do this for itself. 



Much importance has of late years been attached by astronomers to 

 the formal ion of catalogues and charts of stars in the vicinity of the 

 ecliptic, the region of the planetary bodies. The fixed points whose 

 positions are thus determined and mapped, not only serve as points 

 of reference for the places of the moving bodies of our system, but 



2 



