372 SCIENTIFIC AMUSEMENTS. 



tion is, perhaps, one of the least dangerous thiosulphate destroyers 

 on this score. 



Instead of treating paper with albumen, soluble chloride, and 

 silver nitrate solution, it may be sensitised by coating with gelatin 

 bromide emulsion like glass dry plates (Expt. 388) ; or this may be 

 applied after albuminising to produce a glossy surface. Paper so 

 prepared is sold ready for use by most dealers in photographic 

 requisites ; it is usually far more sensitive than ordinary al- 

 buminised paper sensitised by means of silver nitrate solu- 

 tion ; the latter requires exposure to light (preferably direct 

 sunlight) for a long time until the requisite depth of shade is 

 produced; the former generally only requires a few seconds' 

 exposure, after which the picture is developed in much the same 

 way as a glass negative. Gelatin chloride emulsion (gelatin with 

 finely divided silver chloride suspended therein) may be similarly 

 employed. 



Expt. 393. To print a Positive from a Paper Negative. An 

 impression of a fern, fcc., prepared on suitable paper as a negative 

 by the process just described may be transferred as a positive to 

 another sheet of sensitised paper by repeating the process, using 

 the first print as negative to print from. In most cases it is 

 desirable to render the paper negative more transparent by dipping 

 it in melted bees wax or paraffin wax, removing the surplus by 

 pressing it between sheets of clean blotting paper with a hot flat 

 iron. Fig. 202 represents a positive picture of a fern leaf thus 

 prepared from the corresponding negative (fig. 203). 



Expt. 394. Photo Enlargements, or Enlarged Photographs. 

 In order to prepare enlarged photographs, a collodion or dry plate 

 negative is taken of convenient size in the camera (Expt. 387) ; 

 after developing and fixing, &c., this is then used as a slide in a 

 powerfully illuminated arrangement resembling a magic lantern 

 (Expt. 368), the real magnified image formed being received upon 

 a large sheet of sensitised paper. Precisely the same result is 

 thus brought about as in the ordinary processes of printing 

 positives from glass negatives (Expt. 392), excepting that the 

 picture is magnified. Wherever the film of reduced silver is 

 thickest in the negative, least light passes through, and therefore 

 least chemical action is set up on the sensitised paper ; so that the 

 lights and shadows of the finished enlargement are reversed 

 relatively to those of the negative, and a positive magnified 

 picture is consequently obtained. This process is readily applic- 

 able in connection with " carbon " photographs prepared by the 

 chromatised gelatin process (Expt. 398) ; when silver prints are 

 thus prepared, paper treated with gelatin bromide emulsion (Expt. 



