Ch. VII] DEVELOPERS AND LIGHT FOR DEVELOPING 241 



For the most successful use of color screens and different light 

 sources one should have curves of the intensity of the light in dif- 

 ferent parts of the visible spectrum like that for the mazda lamp and 

 sunlight (fig. 36). Then one should know the absorption by each 

 color filter for each kind of light. Knowing these facts and the 

 absorbing and transmitting qualities of his specimens, and the sensi- 

 tiveness of the photographic plates used one could make intelligent 

 selections and reasonably expect good results. 



§ 376. Exposure with color screens. — The color screen naturally 

 increases the time of exposure. It depends on the color and density 

 of the screen. In general the exposure is increased from 2 to 5 times. 

 The increase necessary is usually given by the manufacturers, there- 

 fore each individual worker does not have to find out by experiment. 

 There is plenty of opportunity for the use of his judgment with the 

 different qualities of his specimens (see also § 372). 



§ 377. Developers. — It is best to use the developers recom- 

 mended by the manufacturers of the plates used. The experts 

 employed by the manufacturers have found the best means for devel- 

 oping the plates, and it is safe to follow their advice. One usually 

 has a choice of developers; and as a general statement it should be 

 said that the beginner would be wise to prefer a slow developer, for 

 it allows a greater latitude than a rapid developer. In general a 

 developer containing much bromide works slowly and gives very 

 strong contrasts. Sometimes this is desirable, but often it is better 

 to get the soft effects that come with a small amount of bromide. If 

 one studies the little manuals sent out by the manufacturers there 

 will be found formulae which give the various effects desired. (See 

 collateral reading suggested at the end of the chapter). 



§ 378. Light to develop by. — The light which can be used in the 

 dark room depends upon the sensitiveness of the plates or the printing 

 paper used. The more sensitive the plates or paper the less li.ijht. 

 Furthermore the sensitiveness to the different wave lengths is also 

 important to consider. If the plates are sensitive only to the violet- 

 blue of the spectrum, the dark room can be quite brightly Lighted 

 with red light with entire safety. If isochromatic it orthochromatic 

 plates are used they are sensitive to the spectrum up to and including 



