1890.] Col. Waterhouse — Reversal of image on PliotograpMc plates. 203 



be possible in the present imperfect state of our knowledge of the phe- 

 nomena connected with the formation and reversal of the invisible 

 photographic image, to give any valid opinion — but, although in many- 

 respects the i-esults he had obtained appeared to be in direct opposition 

 to the ordinary conditions of reversal by over-exposure or otl-er abnor- 

 mal action of light, he believed that these reversals were brought abont 

 much in the same way and could be explained by the theories worked 

 out by Capt. Abney some years ago, and generally accepted. 



Reversal in ordinary cases is caused by over-exposure or other 

 abnoi'mal action of light, by the action of certain rays of the spectrum, 

 or by oxidation of the gelatine film, but in this case there is no ques- 

 tion of any of these causes or of any abnormal action so far as exposure 

 to light is concerned. 



If an ordinary gelatine dry plate receive a normal exposm^e a ne- 

 gative image is developed, but if the exposure be unduly prolonged, the 

 light undoes its first action and reversal is produced. In the present 

 instance there was in the developer a substance which Avas capable of 

 liberating halogen by reduction of the haloid silver salt in the dark, and 

 thus the film, although it might only have received a normal exposure 

 to light, was really in the same state under the developer as if the 

 exposure to light were being continued and halogen liberated all over it 

 — as in the case of over-exposure — and the image was reversed by the 

 halogen attacking the parts reduced by the normal action of the deve- 

 loper, while the developer and the alkaline thio-carbaraide completed 

 the reduction of the unexposed parts. During the process the thio- 

 cax-bamide would probably become oxidised and lose sulphur which would 

 m.ost likely combine with the silver and add to the density of the images. 

 The subject, however, required further investigation. 



The only similar case of reversal he had been able to find was that 

 recorded by Carey Lea as caused by the action of hypophosphite of soda 

 on silver bromide films — but though he had tried this substance, and 

 also alkaline solutions of glucose and manna en dry plate films or mixed 

 with the developer, he had not been able to obtain any sign of a reversal, 

 and the reducing action of these substances on the pure silver haloids 

 or gelatine films containing them is not so strong as it is with the thio- 

 carbamides. 



Among the plates exhibited were some of the s|5ectrum which 

 showed reversal only in the ulti'a- violet rays, but he had not been able 

 to fully work out the action of the spectrum with these new reversing 

 agents. 



The results obtained at present left much to be desired in clearness, 

 but he hoped to be able to bring the process to a practical issue. 



