ORTHOCHROMATIC SCREEN.—All orthochromatic plates prepared by any of the methods at present known still remain far too sensitive to the blue and violet rays. It therefore becomes necessary to interpose between the object and the plate a yellow transparent screen which has the power of cutting off a certain portion of the blue and violet rays, and leaves the less refrangi ble with a relative degree of intensity and chemical actinicity more correctly corresponding with the relative action of the eye. Referring to the use and abuse of the orthochromatic screen, Professor Bothamley, who is an acknowledged authority on this subject, says :* " It is obvious that any variation in the tint or thickness of the screen will affect the proportion of blue and violet cut off, and hence will determine the relative activities of the various rays after they have passed through it. From an optical point of view, it is desirable to keep the screen as thin as possible, but the depth of tint of the screen admits of very considerable variation. As the depth of the tint of the screen increases, the proportion of blue and violet cut off increases proportion ally, and the relative action of the less refrangible rays on the plate, or in other words, the rela tive brightness with which green, yellow, orange, and red objects are rendered increases at the same rate. It is obvious, therefore, that it is easy to make a screen so intense that far too large a proportion of blue and violet is cut off ; blue and violet objects will then be too dark, and green, yellow, and orange objects too light. In other words, the resulting photograph will be as incor rect in one direction as the ordinary photograph is in the other. In landscape work a screen too deep in tint causes loss of atmosphere, and with erythrosine plates under these conditions the grass and all other yellowish green objects come out much too light. For landscapes the proper tint of screen is pale lemon-yellow ; for paintings and similar subjects the depth of screen required is usually greater, and is determined by circumstances.
"The great advantage of the screen in landscape work is that it prevents the failure, or, at any rate, want of complete success, which often results from the presence' of a slight blue haze. The details in the distance, which so greatly increase the beauty of many landscapes, are rendered much more satisfactorily, and the more correct values obtained produce a roundness of the foliage, a transparency in the water, and a separation of the various planes of the picture which is rarely, if ever, attained in any other way. This kind of effect is easily seen by looking at any
ordinary view with the naked eye and then through a piece of yellow glass ; the increased round ness of the objects and the separation of the planes is very striking.
" It has been claimed as an advantage for commercial orthochromatic plates that they can be used without a screen, which, of course, is true, but the results obtained are not such as to lead anyone to be enthusiastic about them. It seems as if an attempt were to be made to set up as the ideal of orthochromatic photography a plate which requires no screen. Given that the plate corresponded in sensitiveness with the human eye, that ideal would be realized, but we are some distance from realization at present, so far as gelatine plates are concerned. Moreover, it seems that the use of a screen, with the possibility of varying it to suit the work in hand, or the result which it is desired to obtain, places in the hands of the artistic photographer a power which he would be unwise to cast aside." Carbutt, in his instructions for using his orthochromatic plates, recommends the following method of preparing a suitable color-screen : "Extract the coloring matter from one ounce powdered turmeric in eight ounces alcohol by digestion and frequent shaking for a couple of days, then allow to settle. To two ounces sulphuric ether add 20 grains negative cotton and two ounces tincture of turmeric ; shake until dissolved, and allow to settle ; then coat the piece of thin plate glass, and, when dry, cement another piece of same clear glass over it with Canada balsam ; allow to set for a day or two. The object of using the cemented glasses is that there will be less disturbance of the image caused then when a single glass is used. The color screen can be used either in front or at the back of lens, or placed next the diaphragm, and should be in position when focusing, and when not in use preserved in a box from the light, otherwise the color will soon bleach out.