DRY PLATE PROCESSES. 363 



plate occupying exactly the same position as that into which 

 the collodion plate is subsequently introduced. The lens of 

 the camera is then covered up with a cap to exclude light, 

 and the slide withdrawn, a black cloth being thrown over the 

 whole to exclude any trace of stray light that might get in through 

 chinks and crevices. At a suitable moment the cap is withdrawn, 

 and replaced after a sufficient period of exposure (from less than 1 

 second to 5, 10, or even more seconds in the case of a landscape). 



The slide is then pushed in and the exposed plate removed for 

 development (Expt. 390), which consists in pouring over the plate 

 some reducing fluid which will cause the transformation of iodide 

 of silver affected by light into metallic silver in quantity propor- 

 tionate to the intensity of the action of the light during exposure. 



After the picture has been developed sufficiently, it is fixed and 

 rendered permanent by pouring over the plate (or immersing the 

 plate in) a solution of some compound capable of dissolving away 

 that part of the iodide of silver that has not been decomposed by 

 the developer, without removing the particles of reduced silver. 

 For this purpose a.solution of cyanide of potassium (about 5 to 7 

 parts of salt to 100 of water) or one of thiosulphate of sodium (15 

 to 20 parts of the crystallised salt to 100 of water ; this salt is 

 frequently but unsystematically termed hyposulphite of soda), is 

 generally employed, more especially the former. The final result, 

 therefore, after washing and drying, is that the glass plate is coated 

 with a film of collodion throughout which are interspersed particles 

 of solid opaque metallic silver in greater or less proportion at 

 different spots according as the light has acted there more or less. 



Expt. 388. Dry Plate Processes. In operating in this way 

 many little details have to be carefully attended to, some of which 

 can only be learnt thoroughly by practice ; in any case, special 

 handbooks on photography should be consulted before attempting 

 any experiments with costly apparatus on a large scale, the limits 

 of space in the present work preventing such details from being 

 discussed in full. The collodion plates, prepared as above described, 

 are found to be much more sensitive and to work far better if used 

 fresh and still wet with nitrate of silver solution than if kept and 

 allowed to dry ; accordingly, such plates are always used whilst 

 fresh and moist : but there are many modifications of the process 

 (in some of which films of matter other than collodion are used, 

 e.g., gelatin) where dry plates are employed ; i.e., plates that have 

 been prepared with sensitive material (such as an emulsion of 

 collodion or gelatin and bromide of silver, &c.) and allowed to 

 dry, so that they can be kept for a long time unchanged in perfect 

 darkness. On account of their convenience in use for amateur and 



