" A very effective color-screen and diaphragm combined can be made by dusting the sur face of a piece of glass with French chalk, removing the surplus with camel-hair brush, flow with the turmeric collodion, and allowed to dry. Cut from a thin black card a pattern of the dia phragm to be used, smear a thin coat of gum on one side, and lay it down on the collodionized glass, rub down to insure contact, place between leaves of a book until dry, then remove card diaphragm from the glass and with it the color-screen.
Professor Vogel recommends the following Collodionize a thin plate with Normal collodion IV too parts Aurantia o.4 parts For landscape work 0.2 to 0.3 of aurantia will be found sufficient. The plate thus prepared can be placed either before or behind the lens.
Col. Waterhouse is of opinion that the best screens for orthochromatic work are made of clear thin talc or mica, coated with a cold drying crystal varnish made with benzole and sandarac or dammar colored with annatto or turmeric. Most of the yellow " coal tar" dyes are unfortun ately insoluble in benzole. Varnish is far more structureless than collodion.
His formula is : Turmeric varnish 2 parts Annatto t part Kamala t part The turmeric varnish has a greenish tinge, the annatto a reddish tint, and the kamala (made from the powder obtained from rottlera tinctoria) a neutral yellow. By varying the proportions different tints are obtained.
The real use of a color-screen is to cut off the blue and violet rays, or at least a portion of them. With the orthochromatic plate this is also done, although to a much milder degree sufficient, however, for all ordinary purposes, either in landscape or portrait photography.
An orthochromatic plate is one made more sensitive to the yellow and red rays, and at the same time nearly as sensitive to the blue and violet.
It is this fact that is the cause of its sometimes happen ing that an orthochromatic plate will give the same effect, apparently, as an ordinary one. Where there is no preponderance of the colors to which the former is more sensitive no apparent difference may be detected. Thus it often happens that an orthochromatic plate will in the middle of the day be slower than an ordinary one, but as the sun goes down the light becomes yel lower, with perhaps a slight tinge of red, and the relative speed of the orthochromatic plate becomes greater and greater until it becomes faster by a good deal than the ordinary one. Indeed, it has often been
found possible to make an exposure with an orthochro matic plate so late in the day that an 'ordinary plate would be useless.
We might here consider the question of the advis ability of using a screen for landscape work and it might be answered in this way : When photographing a view containing a large proportion of blue and violet light, and where, as we have said before, it happens that the effect is the same in the two kinds of plates. a colored screen is a decided advantage, but in certain other cases, that is to say, when the sun was low, or the light of a yellow color, a screen would not only be use less but absolutely disadvantageous. The light itself acts as the screen dispensing with its use in the same manner as in copying oil paintings by a yellow light, where a screen is also dispensed with. It is obvious, therefore, that the use of a screen in photographing landscapes must be left to the judgment of the photog rapher. It may be said, however, that it is very rarely necessary, and in almost every case an orthochromatic plate used alone will, by its increased sensibility to yellow or red, give a far better effect and a more truth ful color rendering than An ordinary plate, and this, of course, is more and more apparent the larger the amount of yellow that is included in the picture. Thus, for instance, the effect is most apparent in photographing a woodland scene in autumn when the leaves are turning yellow, and in this case of affairs a screen is absolutely unnecessary. Indeed, its only effect would be to increase the exposure necessary. As we said before, the only occasion for the use of a color screen in landscape photography is when the violet and blue rays have so pronounced an action upon the sensitive film that their representation in monochrome is out of all proportion to the effect of the color value as apparent to the eye. Thus, for instance, in cloud effects upon a blue sky the blue has an action on the film quite as powerful as the white clouds, and consequently there is no contrast visible in the finished picture.