PHOTOGRAPHY. 31 



it, and expose it to the light, you will find the gelatine has 

 undergone a distinct change. First of all, the gelatine has 

 become insoluble in hot or cold water, and in the second 

 place it has become incapable of absorbing water. I have 

 here a sheet of paper (A) coated with gelatine, in which was 

 dissolved potassium dichromate. It has been exposed to 

 light under a negative of a map, and, as I have just told 

 you, it will not absorb water where the lines are printed. 



I immerse it in \vater, place it on this glass plate (B), blot off 

 the excess of moisture, and then roll it with this roller (C), 

 which has been charged with greasy ink. The ink is already 

 beginning to take on the lines ; I run the roller briskly over 

 it once or twice to give more ink to them and to remove any 

 adhering superficially, and we have now got a finished map 

 whose lines are formed of the greasy ink ; this process of 

 reproducing plans I have called the papyrotype process. 



In a simple way I want to show you also that gelatine 

 becomes insoluble in hot water by the action of light when it 

 is in contact with potassium dichromate. I have here a piece 

 of gelatinous paper, which has been exposed under the same 

 negative as before, its surface whilst dry has been covered with 

 a thin layer of greasy ink. Now if gelatine becomes insoluble 

 where the light has acted, when I float this paper on hot water, 

 those parts which have been acted upon by light ought 

 to remain on the paper, and those parts which have not 

 been acted on ought to dissolve, carrying the ink with 

 them. I place the uncoated side of the paper on the boilin-g 

 water, and I notice that an action takes place ; where the 

 light has not acted the gelatine is swelling up, showing 

 that it is absorbing water ; in other words, I see that the 

 lines forming the image are depressed, and the gelatine 

 around is in relief. I pour a gentle stream of water over the 

 surface, and then I wash away the soluble parts by the applica- 

 tion of a sponge. The lines are perfectly distinct, appearing 

 black on a white ground. Both of these properties of chro- 

 mated gelatine, which I have shown you, are utilised in what 

 we call photolithography. The images formed in the greasy 

 ink can be transferred to a lithographic stone and impressions 

 taken in the ordinary manner. This last process is known as 

 the Southampton method for preparing a photographic transfer 

 for lithography. The first method I showed you is certainly 

 equally as effective. It is also on these two properties of gela- 



