366 SCIENTIFIC AMUSEMENTS. 



obtain positive pictures, copies are taken from the first plate after 

 fixing by placing a second sensitised plate immediately behind 

 it, and exposing the two together to light. Wherever the silver 

 deposit is thickest (i.e., where the light originally fell to the greatest 

 extent) less light penetrates through the negative to the other plate, 

 and vice versd; so that the picture finally obtained on the second plate 

 is reversed in reference to the negative, and is consequently normal 

 or positive in reference to the original landscape, &c., photographed. 

 In the printing off of ordinary cartes de visite and photographic 

 portraits generally, this double reversal takes place. A photograph 

 is first taken on glass, which, after development and putting 

 through subsequent processes to render permanent (Expt. 387), is 

 a negative ; positive prints are then taken from this negative on 

 suitably prepared paper by the simple process of placing the paper 

 behind the negative in a glass " printing frame " (something like 

 a transparent slate Fig. 201) and exposing to light until the 

 chemical action has gone on to a sufficient extent, and then fixing 

 the print to prevent further change (Expt. 392). 



"When the object is to produce a negative to be subsequently 

 employed for printing positives a transparent supporting plate 

 (glass, mica, &c.), is indispensable ; but if a positive is intended to 

 be directly produced the nature of the material supporting the film 

 is of no particular consequence; thin sheets of metal, enamelled 

 iron, smooth wood, &c., &c., may be employed successfully. What 

 are frequently called " ferrotypes " are positives directly prepared 

 upon thin sheets of iron coated with a suitable black varnish before 

 applying a collodion film thereto ; the term ferrotype, however, is 

 more properly applied to those photographic processes where iron 

 compounds are the bodies sensitive to light therein employed 

 (Expt. 396). 



The nature of the developing agent varies with that of the film 

 and with the kind of photograph required ; wet collodion plates 

 are usually developed by pouring on to them a solution of pyro- 

 gallic acid or of ferrous sulphate, the reducing action being some- 

 what modified or retarded by the addition of strong acetic acid, &c. ; 

 gelatin bromide plates are usually dipped bodily into a solution of 

 double ferrous and potassium oxalate, or one of pyrogallic acid 

 rendered alkaline with sodium carbonate ; hydroquinone, liydroxy- 

 lamine, and various other organic reducing agents are also employed, 

 mostly in alkaline solution. The "fixing" solution generally em- 

 ployed for wet collodion plates is one of cyanide of potassium, whilst 

 thiosulpliate of sodium is usually preferred for gelatin bromide plates. 



A remarkable circumstance in connection with the latent image 

 produced on a dry plate is that the incipient chemical action set up 



