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X. On the Destruction of the undeveloped Photographic Image. 

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



IT has always seemed that more experiments were required 

 in regard to the destruction of the undeveloped photo- 

 graphic image by chemical or physical agency. In the pre- 

 sent communication I propose to give some instances of the 

 destruction by the former, as it appears they may be capable 

 of throwing light on some of the phenomena which have as 

 yet been only imperfectly explained. 



The undeveloped Daguerrean image, as is well known, can 

 be destroyed by the action of iodine, bromine, or chlorine on 

 the sensitized surface of the plate ; and it can also be destroyed 

 by other agents which might naturally be expected to do so. 

 Perhaps the most remarkable method of destroying the image, 

 however, is by the action of the rays lying at the least-refran- 

 gible end of the spectrum. Draper and others have applied 

 this to obtain an image of this portion of the solar spectrum 

 by submitting a plai:e to which had been given a preliminary 

 exposure to its action. On development with mercury, a ne- 

 gative picture of the red end of the spectrum was obtained, 

 together with a positive picture of the violet end. 



With iodide of silver formed in a film of collodion, the image 

 is known to be destroyed by potassium iodide, probably be- 

 cause it forms a definite compound with the silver image. 

 Sulphuretted hydrogen, coal-gas, and other similar bodies also 

 destroyed the image, by causing a reduction of silver all over 

 the film. This last phenomenon scarcely need be considered 

 here, as it is chiefly dependent on the silver nitrate which 

 kept on the film for sensitizing-purposes. 



Very few, if any, experiments are recorded on the destruction 

 of the image on silver bromide, principally, it may be pre- 

 sumed, because the use of that sensitive salt of silver has only 

 become general within the last few years. The practical pho- 

 tographer well knows the great difficulties that are met with 

 in sensitizing an entirely bromized collodion in the silver- 

 nitrate bath ; and until the emulsion process was introduced 

 in a practical form, experimenting with silver bromide to any 

 great extent was an unsatisfactory undertaking. 



In what is known as the " washed collodio-bromide emul- 

 sion process " we are now able to prepare films containing 

 silver bromide in which neither soluble bromide nor yet silver 

 nitrate are in excess, and experimenting is more easily per- 

 formed even than with silver iodide. 



There has always, however, been one drawback to this pro- 



* Communicated by the Author. 



