In copying colored pictures, oil paintings, etc., a color screen is almost invariably employed, uuless the picture is lighted by a yellow light, which has practically the same effect.
What is the best form of color screen is another question that requires consideration. It is often imagined that any piece of glass coated with a yellow dye in gelatine or collodion will answer the purpose. This is by no means the case. If a glass be used it must be optically plane on both sides. Ordinary glass is liable to be curved either over the whole or in parts, and the effect of such a condition would of course be to throw those parts of the image out of focus.
It would seem to be immaterial where the screen is placed, either before or behind the lens. Many, however, prefer the latter position.
Some years ago we used color screens made of dyed collodion film, pasted over the diaphragm aperture and used between the glasses of the lens. •This is, we believe, a common practice in Germany. Others, again, prefer to place the screen in contact with the plate itself. For landscape work the color should be of a very pale yellow. If too strong it is liable to give an exaggerated effect ; that is to say, the distant parts of a landscape would appear too sharply defined ; they would lose all effect of distance imparted to them by the mistiness that leads aerial perspective to the view.
An arrangement employed at the Weather Bureau at Washington by Mr. A. J. Henry is the following—given in a recent issue of The Scientific American.* The photograph is made through a screen. The one found most effective is that formed of a saturated solution of bichromate of potash inclosed in a plate glass cell having parallel sides. The construction of this cell is shown in the second engraving, in which a a' are squares of plate glass, and b is a ring cut from a glass tube and ground to render its edges parallel and smooth. One side of the ring is perforated and furnished with a stopper. The ring is cemented between the two glass plates with balsam of fir or other suitable cement. The saturated solution of bichromate of potash is introduced through the perforation, and the cell thus made is inserted in a piece of cork, c, which fits over the collar of the camera lens. The proper thickness for the cell is shown in the engraving (Fig. 324); the diameter will of course vary with the size and the angle of the lens. The exposure required for the negative is usually about four seconds.