FouRNiEK d'Albe — On Photography by Reflection under Contact. 99 



disappears, as it is automatically compensated by the grain within tlie paper 

 whieli gave rise to it. Printing-out papers also "give negatives wliich can 

 be used for printing positives on bromide paper without previous toning and 

 fixing. But the exposure has to be very long. 



Bichromated gelatine plates give a good relief by this method, and. 

 unlike the usual printing with bichromate, the more insoluble portion of 

 the gelatine is nearest the support, whether glass or paper. Direct positives 

 may be obtained either by over-exposure (fifty times), or, better, treating 

 ordinary plates or papers with a 10 percent solution of potassium bichromate, 

 and exposing them for the period usual in bichromate printing. The action in 

 this case is as follows : — the bichromated gelatine whicli receives the 

 reflected light is rendered more insoluble than that which adjoins the black 

 areas. Only the latter, therefore, absorb the developer ; and the whole film 

 being "fogged," a direct image of the black portions results. 



The Active Agent. — The very striking results obtained witli bromide 

 paper are attributable to the double absorption undergone by the incident 

 light in penetrating the developed film and returning to the eye of the 

 observer. There appears to be in reality nothing but a purely optical 

 effect. The suggestion was somewliat obvious that some radiation from the 

 pigment of the original, usually, perliaps, intercepted by a slight thickness of 

 air, might inhibit the reduction of the silver salts. This, however, is 

 rendered very improbable by the following observations : — 



1. On impregnating paper with sugar, glycerine, mercuric cliloride, 

 uranium nitrate, and various other substances wliicli do not produce a visible 

 effect, it is found that paper so treated has the same photographic action as 

 the original paper. 



2. The gradation of colours is found to be the same as that obtained with 

 the camera. 



3. Red has no special^ iuiiibiting action. On applying this process to a 

 deep red paper printed with black lettering, a faint negative was obtained, in 

 which tiie black printed whiter than the red. Had red light exerted any 

 inhibiting action, the fog should have been reduced, and the >ed should 

 have appeared wliiter in tlie negative. 



In applying the process described to black and white originals, certain 

 advantages are gained over the ordinary methods withihe camera : 



1. The reproduction is of the exact size of the original ; 



2. The sharpness of definition is only limited by the size of the silver 

 grain in the plate ; 



