13 



pictures of breaking waves, streets crowded with many 

 objects, ships in full sail, a railway train taken broad- 

 side on and running at the rate of sixty miles an hour, 

 and of a flash of lightning, show that the title of the instan- 

 taneous process is well deserved. Excellent portraits are 

 produced in the studio in a single second, and groups out of 

 doors instantaneously. A strong contrast that to the time 

 when the adventurous sitter had his face artificially white- 

 washed, and was compelled to remain in the blazing sun for 

 twenty minutes. 



I should say that the development of these plates differs 

 from that of the wet collodion in the absence of the free silver 

 nitrate from the bath, and in the alkalineity of the developer, 

 but the principle is analogous. To quote Captain Abney, 

 when you have a strongly oxidizing agent in the presence of 

 an alkali, and a silver compound, solid or in solution, then 

 you have ihe last reduced into the metallic state. Such an 

 oxidizing agent we have in pyrogallic acid ; as in the case of 

 the wet plate a restrainer is needed to induce the developer 

 to confine its action to those parts which have been acted 

 upon by light. This restrainer is found among the alkaline 

 bromides. 



The lights and shadows of all objects taken in the 

 camera are, of course, reversed. A positive, as it is called, 

 is therefore printed by the action of light through the original 

 ns.gative on paper prepared with silver chloride. These 

 silver pictures are very beautiful, but liable to fade or decom- 

 pose, and cannot be absolutely relied upon for perpetuating 

 images. Where durability is required, a different process is 

 resorted to. It has been discovered that soluble gelatine, 

 treated with bichromate of potash, becomes insoluble in pro- 

 portion as it is acted upon by light. A suitable pigment is 

 therefore added to the gelatine, and the resulting tissue, as it 

 is called, is exposed under the negative. It is then developed 

 by being placed in warm water, when the black portion 

 dissolves out, and the insoluble picture remains. It is by 

 this process that the Autotype Company has repi-oduced 



