1890.] Col. Waterhouse — Reversal uf image on PliotograpMc plates. 201 



The object of the alteration is to admit of admission fees being 

 made available for the genei'al expenditure of the Society : at present 

 they have to be invested, and the interest only can be spent. 



No objection having been made, the President stated that the pro- 

 posal would be circulated and brought up again at the November meet- 

 ing on the votes of the members. 



Col. Waterhouse exhibited some photographic dry-plates showing 

 a remarkable reversal or transformation of the image from a negative 

 into a positive, caused by the addition to the developer of small quan- 

 tities of thio-carbaniides or sulpho-ureas. 



He said : — These plates, though not so good as could have been 

 wished, might perhaps be of interest from the fact of their being pro- 

 duced in an entirely new and simple manner and with substances which, 

 he believed, had never before been used in photography, though they 

 exerted a very powerful reducing effect on the haloid salts of silver 

 which apparently had not been noticed before and is not pi^oduced by 

 any of the ordinary reducing agents. Apart from the scientific inter- 

 est attaching to any new method of reversing the photographic image, 

 by which light could be thrown on the still unsolved problems connec- 

 ted with the formation of the developed photographic image, a practical 

 process which would enable either positives or negatives to be taken in 

 the camera by the ordinary methods would be of great value in many 

 ways. 



In the course of some experiments, made early in July, with an 

 eikonogen developer, to which a little phenyl-thio-carbamide had been 

 added, he was astonished to find traces of reversal in the darker parts 

 of the picture. Following up this indication he found that with suita- 

 ble proportions of the phenyl-thio-carbamide he was able to produce, 

 at will, more or less complete positive pictures in place of negative ones 

 under otherwise quite normal conditions of exposure and development. 



Further experiments showed that similar results could be obtained 

 with allyl-thio-carbamide, or thio-sinamine, and also, though not so 

 regularly, with thio-carbamide, or sulpho-urea, but not by simple car- 

 bamide, or urea. The latter fact tends to show that sulphur must exert 

 an active influence in bringing about the reversal. 



The thio-carbamides of the alcoholic series are formed by the action 

 of ammonia on the so-called " mustard oils ", or thio-carbimides. 



The phenyl and allyl-thio-carbamides when applied to precipitated 

 bromide or chloride of silver, or to gelatine dry plates or films contain- 

 ing these salts, have no visible action upon them, but if an alkali be 

 added, a darkening and reducing action is set up, even in the dark. 

 With the iodide the action is not so strong. 



