PLATES 215 



process together with a formula for the dyeing bath is 

 given in the appendix, page 270. 



Orthochromatic Plates, Practically all manufacturers 

 make orthochromatic plates in two or three speeds, 

 and some with different degrees of colour sensitiveness. 

 Some brands are dyed in such a way as to be proportion- 

 ally less sensitive to blue, and work as if they were ortho- 

 chromatic plates with a pale yellow screen. They are 

 known under such names as Non-Filter, Anti-Screen, 

 Non- Screen, etc., and can be well recommended when the 

 influence of a pale yellow screen would not be disturbing. 



Plate Speeds. Plates are roughly divided, according 

 to their speed, into Process, Ordinary, Medium, Rapid, 

 and Ultra-Rapid, different makers having different names 

 for these groups. As a rule the speeds marked on the 

 box are only a rough guide to the exposure required. 

 They apply to exposures in daylight, and the exposures 

 in artificial lights, more particularly when using a colour 

 screen, depend on the colour sensitiveness of the plate 

 sometimes to a greater extent than on their speed, and 

 even, a process panchromatic plate might require less 

 exposure for a certain subject than an ultra-rapid non- 

 colour sensitive plate. Rapidity is, as a rule, gained at 

 the expense of fineness of grain, latitude, and resistance 

 to fogging during development. The rapid plates also 

 tend to give less contrast and density. For these reasons 

 the slower the plate that can be used the better. Process 

 or ordinary plates are usually quite fast enough for arc 

 or limelight, but with less powerful illuminants medium 

 or rapid plates must generally be used, as it is most 

 desirable that exposures be cut down to avoid trouble 

 due to vibration and heat. Plate makers are constantly 

 extending the desirable characteristics of slow plates, 

 such as latitude, ease of development, freedom from fog, 

 sharpness of image, etc., to the more rapid kinds, and very 

 few of the rapid brands will be found to offer any difficulties. 



