16 



-emulsion after it has stood for some time. Collodion emulsions are 

 termed washed or unwashed, according as the emulsion is washed 

 before it is poured on the plate or only after the plate has been 



■coated with it. 



In connection with apparatus, the two chief articles of import- 

 ance are the camera and lens. The camera should be as light as 

 possible consistent with strength, and absolutely light tight. "With 

 regard to lenses, the beginner should use at first a sivgle or view lens, 

 proceeding afterwiirds to a rapid symmetrical or rapid rectilinear 

 more suitable for portraiture and instantaneous work. 



The Lecti.rer then described the different forms of lenses used, 

 and the employment of diaphrar/ms or stops for cutting off the 

 outer or peripheral rays. 



The plate having been exposed in the camera, the next process 

 is the development of the image. The development may be carried 

 out in two ways : («) by the acid method generally adopted in the 

 wet-plate process, or {b) by the alkaline methods adopted for the 

 more modern emulsion process. The process of development was 

 illustrated on the screen, and the formulae for different developers 

 were shown by diagrams and tables. Retardation of the develop- 

 ment may be brought about by the presence of an acid, or of 

 soluble bromides or iodides in the developer ; and acceleration of 

 the process by the presence of excess of ammonia or potash. 

 Over-exposure or excess of ammonia causes the phenomenon of 

 fogging in the negative, which is due to the too rapid and general 

 reduction of the silver salt. After development the picture is 

 finally fixed by sodium hyposulphite, which dissolves the unacted-on 



: silver salt. 



The different printing processes were then described, and 

 illustrations given of the process of toning silver prints with 

 solution of gold chloride. Printing in platinum was also sho^vn, 

 this process consisting of two divisions : first, the printing of a faint 

 image by sunlight on the sensitive surface ; and, secondly, the 



'development of the image by warm potassium oxalate. Rapid 

 printing may also be carried out by employing a sensitive emulsion 



■spread on paper instead of glass. In this case exposure to gas-light 



:is sufficient to obtain prints. By employing sensitive emulsions on 



