ON THE PHOTOGRAPHIC IMAGE. 113 



employed. These two facts, indeed, may be held to explain, very nearly, 

 the character of the ordinary collodion process, but they do not explain the 

 "preservative" processes in which the sensitiveness of the film is, within 

 certain limits, retained by the introduction of albumen, gelatine, resin, sugars, 

 or other organic substances, to the numbers of which experience is con- 

 tinually adding. 



For the explanation of the action of these substances, we must recur to the 

 facts already cited in the case of gelatine when used as a size in the direct 

 processes. Thus, too, a plate coated in the ordinary manner with albumen 

 containing iodide of potassium dissolved, will be found, on being raised from 

 out of the silver-bath, not to be opake, and coated with a dense deposit of 

 iodide of silver, but to appear highly translucent and opalescent in its cha- 

 racter, and that, even though the iodide be introduced with a liberal hand. In 

 fact, the albumen is present not merely as a mechanical vehicle for the sen- 

 sitive materials, but can be proved to have combined with those materials, 

 and to play no insignificant part in their photochemical transformation. 

 That this is so, may be at once shown by adding some albumen to a quantity 

 of the ordinary "silver-bath," — say the white of one egg, diluted with l£ 

 ounce of water, added to 40 ounces of bath. The iodide of silver with 

 which the bath was previously saturated will be found in it no more ; it is 

 now to be looked for in the gelatinous precipitate which the albumen has 

 formed. The precipitate is in fact a chemical compound of albumen with 

 nitrate of silver holding in combination the iodide. This is, as might be sup- 

 posed, from what has been said of the albuminate alone, a highly photographic 

 compound. We have stated that a similar compound is formed by gelatino- 

 nitrate of silver and iodide of silver. Citrate of silver, glycyrhizine, and 

 many other bodies share with these substances, and the first two possess even 

 in a far higher degree than they, the property of carrying down in a com- 

 bination — or, so to say, in solid solution — the iodide of silver, and forming 

 with it highly photographic products. 



A hiatus must needs occur in this stage of our inquiry. The sensitive 

 film is exposed in the camera, and in a few instants the invisible image is 

 impressed. We remove it, and our task begins again at a tangible starting- 

 point. The development of the image is the visible evidence that the light 

 has been at work, and a close examination of the nature of this image is the 

 only further key we possess to elucidate the character of the light's action. 



By a comparison of the developed images formed on plates that have been 

 exposed for the correct time to produce a good picture, with such as are 

 produced by the direct action of the light, we arrive at two conclusions. 

 First, a general similarity in the appearance of the various sorts of images 

 by each method is observable ; but, secondly, the deposit in the case of the 

 developed image is far more abundant than that in the direct image. The 

 comparison as regards the quantity of deposit in any two images is one far 

 too delicate to be effected by the balance ; but a method of instituting such 

 a comparison with great accuracy is founded upon the ready conversion of 

 any such images into sulphide of silver, a body transparent and yellow in 

 thin layers, but passing through tones of sepia to almost a black opacity as 

 the thickness is increased. The colour becomes thus a good means of com- 

 paring any two deposits, and the complete conversion of these into the sul- 

 phide is ensured by the use successively of chlorine-water and of sul- 

 phuretted hydrogen. A similar comparative result may be obtained by sub- 

 stituting the chloride of mercury for the chlorine-water. 



Now the deposited images in the case of the processes by development 

 present some points of great analogy to those formed in the direct processes ; 



1859. i 



