[ 200 ] 



XXIX. On the Reversal of the Developed Photographic Image. 

 By Captain Abney, R.E., F.R.S* 



plates V. & VI.] 



IN the Proceedings of the Royal Society f I explained the 

 theory of the reversal of the photographic image on de- 

 velopment, showing that it was due to oxidation when ordina- 

 rily met with. On looking back, however, to the Philoso- 

 phical Transactions for 1840, 1 find Sir J. Herschel gave a de- 

 scription of a similar phenomenon which perhaps might admit 

 of a different explanation. This subject I examined some time 

 ago; and since the whole question seems to have revived in 

 interest, it has seemed an opportune moment to put on record 

 my more recent investigations. I need scarcely say that the 

 fact as to the reversal of the image is any thing but new, as 

 will be seen from Herschel's, Hunt's, and Draper's w T ell- 

 known works. The explanation, however, as far as I know, 

 has been confined to my researches already mentioned ; and 

 these were summarized very briefly. 



Sir John Herschel, in his memoir just referred to, experi- 

 mented on a prepared paper. In article 48 of his communi- 

 cation he says, " A paper endowed with a pretty high degree 

 of sensibility may also be prepared with the following triple 

 solution, viz.: — 1st, acetate of lead; 2nd, hydriodate of potash; 

 3rd, nitrate of silver .... If paper so prepared and darkened 

 in the sun be washed over with a fresh dose of hydriodate, the 

 exposure to sunshine being sustained, it whitens with great 

 rapidity ; and were it practicable (which I have not found it) 

 to ensure precisely the same ingredient-proportions, and the 

 same degree of blackening in the sun to start from, I should 

 not hesitate to propose this as an excellent process for a posi- 

 tive photographic paper." 



This process in Sir J. Herschel's hand was not then uni- 

 formly successful; and it must be noted that here we are 

 dealing with a visible image, and not a developable one. But 

 it will be found that the same argument applies to both, 

 since the visible image and the developable image are of pre- 

 cisely the same nature, varying only in the matter of degree. 



It has been some time known, and more recently has been 

 brought foward by Dr. Angus Smithf, that a slightly aci- 

 dified solution of potassium iodide liberates iodine in the pre- 



* Communicated by tlie Author. 



t Vol. xxyii. pp. 291 & 451. 



\ Since writing this paper, the author has noted a paper in the Philo- 

 sophical Magazine for August 1880, by Dr. Leeds, in which this solution 

 has been investigated. 



