15 



the development of the art. It has been known for centuries that- 

 light acts upon many substances, those whicli show the most 

 marked change being known as salts of silver. That nitrate of 

 silver, or lunar caustic, became darker under the influence of light, 

 was known to the alchemists. Ritter discovered that the lunar 

 caustic was acted upon more by the violet rays than by any other 

 part of the spectrum. Wedgewood took nitrate of silver photo- 

 graphs, but he was unable to "fix" them. Dr. Wollaston took 

 pictures on resins and gums, such as guaiacum. The first real 

 advance, as far as picture-taking was concerned, was made by 

 Niepce. Daguerre sensitised a silver plate with iodine, and 

 obtained an image ; the plate was then exposed in a cupboard to 

 the vapour of mercury, and tlius the picture was fixed, or made 

 pennanent. Whilst Dagxierre was working with silver plates in 

 France, Fox Talbot in England was using paper prepared with 

 iodide of silver. Reade discovered the method of " development " 

 after exposure. Sir J. Herschel suggested glass as a basis for the 

 taking of pictures ; and Scott Archer and Dr. Diamond used a 

 sensitised emulsion of collodion poured on glass. 



The Lecturer then described the manipulation of the wet-plate 

 process. 



In the collodion process, iodized coUodion is poured on a plate 

 of glass, the coUodion dries, and then the glass is dipped in a silver 

 solution. Chemical change ensues, and an iodide of silver is 

 formed on the collodion film, wholly or partially on the surface. 

 Thi.=> is known as the wet-plate process. CoUodion is made from 

 gun-cotton. Nitric and sulphuric acid are poured on cotton-wool, 

 and the cotton-wool is then carefuUy washed in water. The 

 resulting gun-cotton then dissolved in ether is known as coUodion. 



In the emulsion process a definite amount of iodide of sUver 

 is used, and the whole of the material on the surface of the glass is 

 impregnated with the chemical materials. 



The difference between the old wet-plate process and the more 

 modern emulsion processes was iUustrated by the preparation of a 

 coUodion emulsion. In this process definite amounts of silver salt 

 and soluble bromide are carefully mixed together in the previously 

 prepared coUodion ; the plate being coated with this sensitive 



