482 ANNUAL RECOKD OF SCIENCE AND INDUSTRY. 



formed by the acid. The precise details of the operator's 

 manipulation must be managed according to the depth of the 

 etching required. After the operation is alternated several 

 times the plate is finally dried, and the white portions which 

 have not been sufficiently acted upon are cut out with a nar- 

 row saw or with a graver, and the block is then ready for 

 relief printing. 18 A, XXII., 319. 



NEW METHOD OF MICRO-PHOTOGRAPHY FOR MAPS IN WAR TIME. 



A system of micro-photography for maps in war time, and 

 applicable also for the use of engineers {jnd others wdio de- 

 sire to carry a stock of maps and plans with them in a small 

 compass, has lately been devised by Mr. Baden Pritchard, of 

 the Royal Laboratory at Woolwich. 



This method consists in reducing the maps, etc., to a mod- 

 erate size, and photographing them on sheets of white gel- 

 atin. They are then tanned with alum, and washed with 

 collodion and castor-oil. The maximum size of the map is 

 six square inches. Fifty or sixty sheets can be pressed into 

 a space of half an inch in thickness. Officers are instructed 

 to have a collapsible dark chamber, into Avhich, when not in 

 nse, the photographs can be slid. They can then be exam- 

 ined like transparent spectroscoj^ic slides. It is proposed to 

 adopt this arrangement at once for the service of the British 

 army. 3 A, June 24, 1876, 808. 



PHOTOGRAPHIC COPYING-PAPER FOR DRAWINGS, ETC. 



A very simple French copying-paper affiDrds copies in 

 w^hite lines on a blue ground of drawings pressed in contact 

 W'ith it by means of a glass plate, after ten minutes' exposure 

 to sunlight. The pictures are then fixed by simply washing 

 them in water. According to analyses by Professor Schwarz, 

 the ground consists of TurnbuU's blue, formed, doubtless, 

 from the action of ferricyanide of potassium on ferrous ox- 

 alate, formed by the reduction of ferric oxalate by the light. 

 He recommends for the preparation of a similar article float- 

 ing paper on a mixture of 100 cubic centimeters of a solution 

 of 31.7 grammes of red prussiate of potash in 500 cubic cen- 

 timeters of water, with 300 cubic centimeters of a solution 

 of ferric oxalate, prepared by saturating 9.45 grammes of 

 crystallized oxalic acid with the precipitate formed by am- 



