This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
The relative brightness of the colors of an object as represented in an ordinary photograph is very different from the relative brightness of the same colors as seen by the eye. This is due to the fact that all photographic plates including the very best of the so-called Orthochromatic (Hammer's), are enormously more sensitive to the blue and violet than to red, yellow and green.
In the early days of these plates, some persons had the idea that Ortho plates could not be used at all without a screen - try a few and see.
There is some advantage with Orthochromatic Plates in color rendering without the screen, if the violet and blue do not predominate. All plates have excessive sensitiveness for violet and blue. Orthochromatic Plates show this tendency also. The use of the screen is to cut off the excess of the actinic light.
Much has been said about adjusting the screen to the color sensitiveness of the plate; of course, this is important in such exact work as three-color printing necessitates, but with landscape and all ordinary objects that are to be photographed, less exactitude is permissible.