476 ANNUAL RECOKD OF SCIENCE AND INDUSTRY. 



plates, prepared up to the sensitive stage, are rendered capa- 

 ble of being kept in that condition by washing them thor- 

 oughly with water, then applying some preservative solution, 

 as of tannin, gum, etc., and drying them. Although some of 

 these processes in careful hands have been made to aiford 

 very satisfactory results, they are upon the whole inferior to 

 the wet process in rapidity, certainty, and general excellence, 

 in spite of the excessive care and labor required in the prep- 

 aration and development of the plates. Another plan of 

 relief from the silver solution, which naturally suggested it- 

 self at an early day, was the formation of the sensitive silver 

 salts in tlie collodion, before pouring it upon the glass plate, 

 by introducing into it the silver nitrate and the necessary 

 iodides and bromides. But, after trial, this direct method 

 was regarded for a long time as impracticable, mainly be- 

 cause the iodide and bromide of silver could not be made to 

 form in a sufficiently finely divided condition to remain sus- 

 pended in the collodion. At last, however, the so-called 

 washed collodion emulsion process has been brought to such 

 a degree of perfection that all the manipulations hitherto 

 found necessary with the baths, washing-trays, preservative 

 solutions, etc., seem to be reduced to the minimum of the 

 one single, simple operation of pouring a sensitive emulsion 

 of the haloid salts of silver with collodion upon the glass 

 plate, and allowing the film to dry. All the articles required 

 are already to be had in the market. The jDlates thus pre- 

 pared are said to be of the best quality, of great certainty, 

 and of absolute uniformity, equal in sensitiveness to those 

 prepared by any dry process, and afix)rding a fine film, free 

 from granulation, even under the microscope, and, by regu- 

 lation of the development, adapted to the production of neg- 

 atives or positives, as may be desired. Besides the freedom 

 from custom-house annoyances that this process guarantees 

 to the tourist, it renders him at all times, as far as the size of 

 his plates is concerned, entirely independent of the size of 

 the receptacles for the various solutions. The first success- 

 ful experiments in this direction were made by Bolton and 

 Sayce, two amateurs of Liverpool, about ten years ago. 

 They found that by omitting the iodide, an emulsion of bro- 

 mide of silver with collodion could be formed, suitable for 

 photographic processes. The conditions most favorable to 



