36 M. C. Lea — Image Transference. 



was then placed between the leaves of a closed book with one 

 end projecting. The book was then placed in the light (very 

 faint sunshine), and the prepared paper was gradually drawn 

 out so that different portions received progressive exposures 

 from 400 seconds down to 3 seconds. A number of marks were 

 made with HC1 diluted, parallel to the end of the paper, so that 

 to each mark corresponded a different exposure from 3" to 

 400". The papers were then placed in nitric acid, washed and 

 developed. These .pieces are before me as I write, and it is im- 

 possible to say by the appearances which portion received the 3 

 seconds, which the 400. The marks are equal in strength on 

 each paper from one end to the other. The salts used were : 

 silver benzoate, tungstate, phosphate, pyrophosphate and tar- 

 trate. The short exposure gave a sufficient basis for develop- 

 ment, the longer effected nothing more. 



The object of this series of papers has been to offer a new 

 explanation of the nature of the latent photographic image, and 

 to show that it consists neither of the normal silver haloid 

 physically modified, nor of a subsalt, but of a combination of 

 normal salt and subsalt. That the subsalt loses in this way its 

 weak resistance to reagents, and acquires stability, thus corre- 

 sponding to the great stability of the latent image, which, 

 though a reduction product, shows considerable resistance to 

 even so powerful an oxidizer as nitric acid. 

 . Further, that this combination of normal salt and subsalt, 

 which constitutes the material of the latent image, can be ob- 

 tained by chemical means, and wholly without the aid of light. 

 That the forms of these photosalts, as I have ventured to name 

 them, which correspond to the material of the latent image are 

 either colorless or nearly so, but that other forms, possessing 

 beautiful and often intense coloration, also exist. With the 

 chloride some of these brightly colored forms show a ready 

 tendency to reproduce color, in some cases with well marked 

 and beautiful tints. So that we have here an approach to the 

 solution of the problem of obtaining images of objects in their 

 natural colors from a quite new direction, and probably with 

 better hopes of an eventual complete success than by any of 

 the older methods. 

 Philadelphia, May 17, 1887. 



