202 Captain Abney on the Reversal of 



after washing be treated with a solution of potassium iodide 

 and then be exposed to an image in the camera, that, after 

 dipping in the silver-bath and developing, a positive image is 

 obtained. It matters not whether the potassium iodide be alka- 

 line, neutral, or acid, the same effect will be noted; also that 

 there is no difference if, after treatment with the potassium 

 iodide, the plate be washed or not, the reversal of the image 

 will still be shown. In this case the iodine is liberated as 

 before, but the action is increased by the access of oxygen from 

 the air ; in fact it is a mixture of effects. 



If potassium bromide or any simple bromide be substituted 

 for the iodide, the same result obtains. Silver iodide, if pre- 

 pared with an excess of soluble iodide, or if, after preparation 

 with excess of silver, it be treated with a soluble bromide, is 

 insensitive to light ; and the explanation of this perhaps may 

 be found in the fact already stated. 



It has been usually held that a soluble iodide, such as potas- 

 sium, can destroy an invisible impression made by radiation ; 

 but this is not the case if it be treated with the iodide in 

 the dark. If, however, any iodide, such as cupric or ferric, be 

 employed, which readily liberates an equivalent of iodine, the 

 destruction is accomplished in the dark. The least favourable 

 iodides for such destruction, as I have already shown*, are 

 the monads. 



If a plate prepared with silver iodide have a preliminary 

 exposure given it, and then be exposed for a considerable time 

 to the image formed in the camera, a reversal of the image 

 will take place as before. If, however, such a plate, after 

 washing, be treated with an aqueous solution of pyrogallic acid, 

 potassium nitrite, or any other deoxidizing agent, such reversal 

 of the image will not be obtained ; nor will it if it be exposed 

 in a cell containing such a substance as benzene, or if exposed 

 in dry hydrogen. From this we learn that, to obtain reversal, 

 oxygen must be present in some form or another, and that, if 

 a substance readily taking up oxygen be in contact with the 

 silver-salt, a reversal cannot be readily obtained. 



An interesting corroboration of the above statement is to be 

 found in the treatment of an exposed plate in a cell containing 

 a dilute solution of permanganate of potash, bichromate of 

 potash, or hydroxyl, when it will be found that the reversal 

 takes place with the greatest facility. The same reversals 

 may also be obtained by using any of the mineral acids in a 

 diluted formf . 



* Photographic Journal, 1878. 



t It must, however, be remembered that the solutions must be very 

 dilute, or the whole effect of the preliminary exposure will be destroyed, 

 since these oxidizing agents are active in the dark, but act more readily in 

 the light. 



