GENERAL PRELIMINARY PREPARATIONS 199 



diffusing medium, such as the latter, rather than plain glass, 

 as the light is thereby better distributed over the working 

 area. The amount of light that is permissible depends entirely 

 on the plates to be used, but in any case it is not advisable to 

 reduce the illumination of the room to too great an extent ; 

 not only does this entail some strain on the worker, but it is 

 also much more difficult to observe the progress of the plate 

 during development. Anyhow, whatever plate is used, means 

 should be taken to shield it as much as possible from the dark- 

 room light during all the operations. For bromide-printing 

 processes or lantern-slide making, one thickness of orange 

 glass will be sufficient, as in neither of these processes are the 

 emulsions so sensitive. In working with panchromatic plates, 

 which are sensitive to practically the whole spectrum, recourse 

 must be had to one of the new green safe lights. Nearly all 

 photographic plates are less sensitive to a narrow band in the 

 spectrum in the region of the green, and it has been possible, 

 by making a dark-room light of such a colour that it only 

 transmits green in this particular region, to work in comfort 

 with plates that cannot be safely exposed to any red light. 

 Such green colour-screens can now be obtained commercially 

 either from Messrs. Wratten & Wainwright or from Messrs. 

 Sanger, Shepherd & Co., and these are in practice by far the 

 best illuminants for the dark-room. They may be obtained 

 in practically any size to suit the particular form of lamp in 

 use. It is advisable to provide against their being subjected 

 to any considerable amount of heat, so that the lamp should 

 be one in which the illuminant is not too close to the screen. 

 In some forms of lamp it is possible to place a glass cell in 

 position, and to fill this cell with a solution composed of dyes 

 which transmit such light as is required. The advantage of 

 such a light is that there is less general absorption and the 

 quality of the light can be exactly varied at will. One form of 

 these is a cylindrical glass vessel in which an electric lamp is 

 suspended, so that it is only necessary to fill the vessel with a 

 coloured solution to obtain an efficient means of illumination. 

 Alternatively, gelatin or collodion films may be dyed in the 

 manner to be described for making coloured filters, but these 

 require using with care since they are rather subject to change as 

 the result of either overheating or prolonged exposure to light. 



