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.
When, a few years ago, Orthochromatic plates made their initial appearance before the photographic public, the venture was looked upon by the average amateur merely as a somewhat interesting experiment. The importance of the discovery which rendered such a product possible was not realized and although conceded to be of much interest, yet it did not seem to materially concern their work. The professional photographer simply ignored them.
714. The Orthochromatic plate, however, has steadily gained in favor and the many "conventions" and "salons" which have been organized in later years have greatly served to demonstrate the advantages to be gained from their use, until, at the present time, there is no plate maker of note throughout Europe or America who does not supply such an article in one or more "brands."
715. The principles of orthochromatism are now (thanks to the many writers on that subject) fairly well understood among photographers, but to the ever increasing army of new workers a brief explanation is essential to intelligent use, while even to those who are further advanced, a concise account may not be out of place.
716. Briefly : "White" light, so called, is a complex mixture of 6even different colors which are respectively red, orange, yellow, green, blue, indigo, and violet, and these colors arranged in the above order constitute the "visible spectrum "(or image) of that light when it is observed through a glass prism. Besides this there is another region known as the ultra-violet, which is totally invisible to the human eye, but to which the photographic plate is very sensitive - whether it be "ordinary"or "orthochromatic"
717. All objects in Nature appear colored because they reflect more of some one color than another - those colors which they reflect less being absorbed by the object itself.
718. The ordinary photographic plate is not sensitive to all of those colors, but only to a very limited number - in fact only three - the blue, indigo, and violet; and also to the ultra-violet. The result is, that the red, orange, }yellow or green have no action on the plate at all, and hence are represented as black. The print then is altogether untrue to the original in all save shape - a dark violet object for example, being represented as a bright white; while, on the contrary, the brightest of all colors - yellow - will be black.
719. The Orthochromatic plate is the result of the addition of certain dye stuffs into the sensitive coating, which makes the plate capable of being impressed by other colors, viz., the yellow and green.
720. Although the plate is now sensitive to yellow and green, yet it is still much more sensitive to blue, violet, and the invisible ultra-violet, and, to render the colors of Nature in anything like their relative brightness to one another, it is necessary to tone down those colors which act too strongly, so that the others may have a chance to impress themselves with sufficient strength. This is accomplished by the employment of a colored screen, or more properly speaking - a light-filter.
721. The first object of such a filter is to cut off entirely the invisible ultra-violet, so that it may no longer impress itself. Next, it must absorb a definitely measured amount of blue, indigo and violet lights, so that they do not act any more strongly than is necessary to show objects so colored in their proper brightness. If the filter should tone down these hues too much, then the result would be just as far wrong the one way as it was the other It is therefore obvious that a filter adjusted to one make of orthochromatic plate, will not give equally good results with some other.
722. The Seeds "Chromatic Curve" filter is adjusted by careful spectroscopic and other measurements, so that it conforms exactly to the needs of the Seeds "ortho" plate, the combination giving results which reproduce with the utmost fidelity the varying luminosities of the object photographed.
723. The Seeds "Compensator" filter is very much lighter in color, and - as its name implies, is for use in compensating the errors of the plate - its duty is to absorb entirely and completely the invisible ultra-violet, and slightly subdue the visible violet. (This filter does not possess the measured adjustment of the "Chromatic Curve.")
724. These filters are arranged for the utmost convenience in every day use, being manufactured of optical plane glass, held in a handsome metal cell with adjustable clasps to fit slightly varying sizes of lenses.