This section is from the book "Modern Photography In Theory And Practice", by By Henry G. Abbott. Also available from Amazon: Modern photography in theory and practice: A hand book for the amateur.
In Chapter III the attention of the reader was called to the fact that light is made up of several colors, red, yellow, orange, green, blue, indigo and violet and that blue, indigo, violet and green rays act more quickly than do the red, yellow and orange ones. It was also explained that if we expose a plate just long enough for the blue and violet rays, then we will probably under expose it for the red and yellow ones and that to have a perfect negative we must in some way correct the color value by reducing all to a common value, or as nearly so as possible. This correction is partially effected in two ways, either by the use of Isochromatic or Orthochromatic plates and color screens, or by the use of color screens in conjunction with ordinary plates. These color sensi tive plates in conjunction with color screens, or ray filters, as they are sometimes known, are very useful in photographing flowers, clouds and other objects and scenes where it is advisable to have the proper relative color values. In these days of quick-working lenses, shutters and plates, we find a greater percentage of cloud effects than formerly. The photographing of clouds, by themselves or in conjunction with a marine or landscape, is highly inter-esting and the addition of clouds to the ordinary view adds to it at least fifty per cent in value from an artistic standpoint. Almost any lens will answer for photographing clouds by themselves but it is not every lens which will bring them out in conjunction with a land scape. It is not absolutely necessary to use either color sensitive plates or color screens in order to insure the production of clouds in your landscape, for a proper lens and fast plates will very often result in first-class cloud effects, as the accompanying illustration proves; but the cloud effects will certainly be enhanced by the use of color sensitive plates alone and a greater improvement made by the use of such plates in conjunction with color screens.
Color screens or ray filters are made in several ways. The coloring matters usually employed are Auramine, Bichromate of Potash and Picrate of Ammonium for color values on sky negatives and subjects whose tones incline to blue, indigo, violet and green. These coloring matters are employed in several different ways. Some color screens are made of mica, colored with one of the above ingredients, others consist of glass cells containing the coloring matter in solution and others consist of glass plates. The great objection to the color screens on the market is that as a rule they are too deep or strong in color. This strength not only prolongs exposures from twenty to forty times but the results are unnatural pictures. We have seen color screens in the form of cells containing a saturated solution of bichromate of potash. Such cells make necessary an exposure forty times the length of the normal and the result is an intensely black sky with white clouds, a thing never seen in nature. The manufacturers claim that the screen is built on scientific principles and tested by the spectroscope and found correct. This may be true and if it is, then we do not want scientific effects, for they are certainly unnatural. While the clouds stand out beautifully the background is certainly unlike anything in nature. For this reason care must be exercised in the selection of color. A saturated solution of picrate of ammonia produces a light yellow which requires but very little further reduction if used with lacquer or gelatine but if used as a solution in a glass cell it can be slightly reduced. We have used a bichromate solution which consisted of one dram of saturated solution to 4 drams of lacquer and found that that was fully strong enough for the darkest screen. Our best results have been from screens made of lantern slide cover glasses, coated with lacquer which was colored with picrate of ammo-nium. A saturated solution of the picrate was made by mixing with the lacquer. Three different screens were all made very light in tone and where a darker tone was desired one glass was placed in front of the other. Fig. 45 illustrates a front and top view of the holder for these screens. The body of the holder is made of hard rub-ber, turned from one piece, it being square in front and turned down at the back, leaving a cap to fit over the front of the lens. This cap is lined with a piece of chamois leather. The front has a frame made of light brass with the top quarter of the frame removed. This frame sets out far enough from the rubber body to admit of the insertion of two screens, one in front of the other, with a bar between them. The solution side of the color screen is, after testing and being found correct, to be covered with another cover glass and the two held together by means of a binding of tin foil. A wire loop is fastened to the top, so the screens can be easily removed. Instead of lacquer, gelatine may be used. A saturated solution of the picrate is made and to each ounce of this solution 15 grains of gelatine are added and then alcohol enough to bring it to the required shade. Before using either the picrate or any other solution, it should first be filtered through a clean piece of fine linen to remove any lumps or grains. If preferred, the various density of plates may be secured by first starting with a very light color, darkening by additional applications of the lacquer or gelatine but our experience is that a light film is better than a heavy one. Another method is to put unexposed lantern slides into a hypo bath, clear them up thoroughly, wash and then apply the solution to the film. The plates must be fresh to accomplish good results in this way.
The Government Weather Bureau at Washington employ the solutions in glass cells. Fig. 46 gives an idea of how these cells are made. The frame is made of cork, although wood or hard rubber might be substituted. A ring of glass cut from a tube about the circumference of a lens and with the sides ground perfectly flat with emery is cemented between two thin pieces of plate glass. This ring has a small hole at the top for the insertion and removal of the solution and this hole is stopped with a small metal plug. The illustration to the left shows the completed screen in place on the lens. The glass ring is cemented to the square glasses by means of Canada balsam. If alcohol is used as a base for the solution much difficulty will be encountered in preventing the cell from leaking and silicate of soda or soluble glass will then be found better as a cement. The cell is filled and then fits, friction tight into the square in the cork frame. It is much easier to make the cell in this form than where round glasses are fitted into a metal cylinder and less chance for leakage. Its other good points are that several different densities of cells can be used in the one frame and one can be pushed out and a new one inserted without trouble. The ring of glass must be the same width all around, otherwise the screen would be denser in one place than another on account of the excess of fluid. The Weather Bureau people employ a saturated solution of bichro-mate of potash. They secure thereby very showy but very unnatural negatives and prints. If they were to reduce the density much more natural effects would follow.
Other colors can be used with equal advantages on certain kinds of color work. Emerald green can be made into a solution and used in the liquid form or to coat the plate by means of lacquer. Methyl violet may also be used to advantage. These screens are useful in copying water color and oil paintings. In order that the action of screens may be fully understood by the amateur let us put it in a little different form.
A Yellow Screen cuts out in the negative the blues and indigoes, leaving clear glass in their place.
A Green Screen cuts out in the negative the reds and leaves clear glass in their place.
A Violet Screen cuts out in the negative the yellows and leaves clear glass in their place.
The above refers to screens of dense color and lighter screens of course have similar effects, though in a less marked degree. From this it will be seen that if a yellow-orange screen of considerable density be used there will be clear glass where the blue of the sky would be and the consequence is that the resulting print would appear as white clouds on a black background.
Flash Light. A Bachelor's Reverie. J. E. Green, Chester, Pa.