On the Reversal of the Developed Photographic Image, 201 



sence of light, though it remains unaltered in the dark for 

 considerably long periods. If, then, paper impregnated with 

 a silver-salt be blackened by light and be then treated with 

 potassium iodide, we have the exact explanation of Sir John 

 Herschel's experiment, presuming the paper be slightly acid. 

 "When iodide of silver is exposed to light in the presence of a 

 neutral solution of silver nitrate, we have this acidity produced, 

 the iodine liberated from the silver iodide in its conversion to 

 subiodide combining with the silver nitrate and liberating nitric 

 acid, probably with the formation of an iodate. This is true 

 more especially when the paper is not absolutely desiccated ; 

 for when desiccation is perfect the iodine might not combine. 

 Hence may arise the uncertainty to which Sir J. Herschel refers. 

 If paper so prepared be kept damp by any means, the reaction 

 will invariably take place, and iodine from the potassium iodide 

 will be liberated and combine with the semi-metallic silver on 

 the paper to produce silver iodide : in other words, the black- 

 ened surface will bleach. 



The experiment may be tried in a variety of ways.' The sim- 

 plest, perhaps, is to salt ordinary unglazed paper with a 10-per- 

 cent, solution of common salt, and when dry to float it on a solu- 

 tion of silver nitrate of about the same strength, and then to 

 dry and expose it to the daylight to blacken. TThen the black- 

 ening is produced, if the paper be slightly washed and then be 

 treated with a 5-per-cent. solution of potassium iodide (slightly 

 acidified with nitric acid) in the dark, and while still damp 

 be exposed beneath a negative to the light, it will be found 

 that those portions beneath the transparent portions will 

 rapidly bleach, and we shall have a negative image instead of a 

 positive, but reversed as regards right and left. 



The same experiment may be repeated, substituting potas- 

 sium bromide for the iodide, and the same results will be ob- 

 tained. It may be asked if any metallic iodide or bromide will 

 be effective ; and to this an affirmative answer may be given : 

 but the use of acid is not necessary in all cases. Those metals 

 which form two iodides or bromides must be used extremely 

 dilute, or the bleaching will take place in the dark — that is, 

 supposing the highest type of bromide or iodflte be employed. 

 Thus a strong solution of zinc iodide may be used and aci- 

 dified, whilst a very dilute solution of ferric iodide must be 

 used. 



This, then, is the explanation of the reversal of the visible 

 image; audit now remains to show that the same action takes 

 place in the invisible image. 



It is well known, if a plate be prepared with silver iodide 

 by the ordinary wet process, be briefly exposed to light, and 



