238 Intelligence and Miscellaneous Articles. 



was then charged with nitroglycerine to a height of 4J inches. It 

 was then closed for a distance of i^- inch by means of an iron 

 screw plug, having in its axis a tube, which received on the one side 

 the powder, on the other the fuse. The effect was complete, the 

 block broke into four large and ten to twelve small pieces, and the 

 carriage on which it rested was broken. — Comptes Rendus, July 17, 

 1865. 



ON THE NATURE OF THE INVISIBLE PHOTOGRAPHIC IMAGE. 

 BY M. CAREY LEA, PHILADELPHIA. 



Some experiments in which I have lately been engaged seem to 

 me to finally settle the long-contested question as to the nature 

 of the invisible photographic image, and I hasten to send a very 

 brief description of them, hoping it will be in time for your July 

 number. 



The view that the change which takes place in an iodo-bromized 

 plate in the camera is a purely physical one, that no chemical de- 

 composition takes place, and neither liberation of iodine nor reduc- 

 tion of silver, has obtained a pretty general acceptance. But lat- 

 terly it has been opposed by two distinguished photographers, Dr. 

 Vogel and Major Russel. The former affirms that iodide of silver 

 is never sensitive unless there is a body present capable of taking 

 iodine from it under the influence of light. And Russel believes 

 that the developed image is chiefly produced at the expense of the 

 silver haloid in the film. 



The following experiments seem to me to decisively close this con- 

 troversy in favour of the physical theory. 



Experiment 1. — If the iodide or bromide of silver in the film 

 undergoes decomposition in the camera, and still more, if the deve- 

 loped image is formed at its expense, the film of iodo-bromide must 

 necessarily be greatly consumed in the development under the dense 

 portions of the negative which it has contributed to form. 



To settle this point, I exposed and developed an iodo-bromized 

 plate in the ordinary manner. Then, instead of removing the un- 

 changed iodide and bromide by fixing in the ordinary manner, I 

 took measures to remove the developed image without affecting the iodide 

 and bromide. This I succeeded in doing with the aid of a very weak 

 solution of acid pernitrate of mercury. Now if the iodide, or bro- 

 mide, or both had been in any way decomposed, to form, or aid in 

 forming the developed negative image, when this came to be removed 

 there should have been left a more or less distinct positive image, 

 depending upon varying thicknesses of iodide and bromide in the film, 

 much like a fixed negative that has been completely iodized. Nothing 

 of this sort was visible, the film was perfectly uniform, just as dense 

 where an intense sky had been as in those parts which had scarcely 

 received any actinic impression, and looking exactly as it did 

 when it first left the camera and before any developer had been 

 applied. 



