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.
635. For developing use any of the normal developers, or the Contrast Developer. The two may be mixed in proportions to suit or used in two trays alternately. For producing the greatest contrast, with absolutely clear shadows (black and white), use the developer for line work given among the developers.
Cramer's Transparency Plates, On Clear Or Ground Glass. Coated with a specially prepared slow emulsion, producing the very finest transparencies or positives on glass. Develop with Transparency developer given among the developers.
Cramer Lantern Slide Plates. On first quality, thin crystal glass, producing rich, brilliant slides with absolutely clear glass in the highlights.
638. Lantern Slides can be made from negatives by copying in the camera or by contact. The time of exposure when printing by contact depends on the source of light and the distance of the light from the negative. Under a negative of ordinary intensity at a distance of three feet from the light of an ordinary fish-tail burner, or an incandescent light, a few seconds' exposure will be sufficient. The proper time required can easily be ascertained by a few trials. If the negative is very dense the distance from the light should be shorter, if the negative is thin the distance between the negative and the light should be increased.
639. For developing use Transparency developer, given among the developers.
Comparative Speed Of Cramer Plates. (Approximately.) - If the Crown requires 1 unit of time, Banner X will require 11/4 units, Blue Label will require 1 1/2 units, Anchor will require 2 1/2 units, Instantaneous Isochromatic will require 1 unit, Medium Isochromatic will require 1 2/3 to 2 units, Slow Isochromatic will require 8 to 10 units, Contrast will require 10 to 15 units, Transparency and Lantern Slide 10 to 15 units, Double coats according to brand.
Panchromatic (Bathed) Plates. It has long been known that the introduction of dye stuffs to the sensitive emulsion during the process of manufacture does not give as high a color sensitive effect as may be obtained by bathing the finished plate in a dye bath. The great uncertainty, however, dependent upon this mode of treatment, and the necessary skill and apparatus required for successful results, has precluded its adoption save by isolated individuals. The Cramer Company have recently installed complete apparatus for the production of plates and are supplying plates sensitized for any region of the spectrum.
Cramer's "Paniso" (Bathed) Plate. - This plate is sensitive to the entire range of the spectrum from the extreme infra-red to the extreme ultra-violet and must be handled and developed in total darkness. That total darkness is a necessity, is simply a tribute to the color sensitiveness of the plate.
The Paniso is primarily suited to the needs of scientific and three-color workers, but they are also particularly well adapted, when used with their proper ray filter, for the commercial photographer, or for anyone who is engaged in the photographing of colored objects.
By their use deep reds and greens which are otherwise either unattainable, or imperfectly rendered, are represented with values absolutely unapproached by any other plate. For landscape photography by advanced workers their use with an extremely light ray filter gives absolutely correct color values.
As has already been stated, the Cramer Company prepare (on order) plates sensitized for any spectral color, but in the use of all bathed plates due attention must be paid to the fact that plates so prepared have but limited keeping quality, so should not be stocked in quantities. Such plates are not handled by dealers, but must be ordered direct from the factory.
The introduction of the Paniso among scientists reaped an instantaneous success, for in the hands of Professor Hale of the Carnegie Solar Observatory, were obtained results which shed revolutionary light upon the solar theories.
In the exacting field of photo-micrography, these bathed plates stand for the achievement of the highest possible results. No object is so stained that it cannot be reproduced with full detail.
Color Screens (or Ray-Filters). - To appreciate thoroughly the use and value of a ray filter in conjunction with an isochromatic plate it is necessary to give some consideration to the imperfections of the ordinary photographic plate.
All ordinary photographic plates are color blind and therefore represent wrongly the relative brightness of different colors, a dark blue photographing as very light, while a bright yellow is copied as being very dark. Or supposing one is exposing upon a green field in which are clusters of bright yellow flowers, the flowers would be invisible on the finished print (because the plate is not sensitive to yellow), and the whole effect of the scene would be lost.
An isochromatic plate is a photographic plate which has (by changes in manufacture) been made sensitive to the yellow and green. With such a plate a patch of bright yellow is represented as being of about equal brightness with a patch of green or blue, and this is as far as the sensitive plate is capable of.
Now in order to obtain a record of the yellow flowers as being brighter than the green grass, it is necessary to make use of a ray filter which is used in front of or behind the lens, and whose purpose is to dim down or hold back the blue and violet rays to which the plate is too sensitive. "When any ray-filter is made use of it slows down the working speed of the plate, because it has filtered out some of the light rays which are falling upon its surface, therefore more time must be given to the exposure.