1116 Transactions of the American Institute. 



iodide of silver film, which, notwithstanding it was invisible to the 

 eye, might manifest itself when the plate was exposed to the 

 influence of other substances. It was soon found that this silver, 

 separated from the iodine by the influence of light, had obtained a 

 great aflinity for mercury, and, consequently, a strong tendency to 

 combine with mercurial vapors not possessed by the iodide of silver 

 when the vapors were not heated above 180 to 200 degrees Fahren- 

 heit. This iodide of silver, then, not combining with those vapors, 

 the consequence was that, when such a plate had been exposed to the 

 luminous rays in the camera for a few minutes only, and on which 

 nothing was yet visible, exposure to mercurial vapors of some 180 

 degrees Fahrenheit would not aftect the silver plate wherever it was 

 protected by the film of iodide of silver, Wt only where some silver 

 of this film had been set free by the action of light. The amount 

 of deposit of mercurial vapor was also found to be proportional to the 

 amount of silver set free, which, again, was proportional to the pre- 

 vious intensity of the light acting on diflferent parts of the plate. 



The deposit of mercury, or rather amalgam of mercury and silver, 

 thus formed on the surface of the protecting film of iodide of silver 

 constituted the picture. A microscope, when of sufficient magnifying 

 power, detects the amalgam to consist of minute globules, very close 

 together in the high lights of the picture, less close in the less illumi- 

 nated portion, very sparsely distributed in the shadows, and altogether 

 absent in the blanks, which consisted of pure iodide of silver. 



After the reading of Prof. Yanderweyde's paper, several mem- 

 bers entered into an animated discussion upon the chemical and 

 mechanical action of forces in the production of the photographic 

 image. 



Mr. Mertz related an interesting case in which he had produced 

 a definite image of a solar eclipse on a collodion film without the 

 use of any developing agent. The image was quite distinct when 

 the plate was taken from the camera, 



Mr. J, Johnson gave an account of some recent experiments on 

 actinic light in photographic printing and the growth of plants. 



Mr. H. T. Anthony remarked that he had found in an extensive 

 photographic printing establishfiient, the amount of gold and silver 

 remaining in the finished prints were about equal. This was 

 determined by the treatment of the residues, and recovery of the 

 metals. 



The section then adjourned to the first Tuesday in May. 



