This section is from the book "The Barnet Book Of Photography", by Herts Barnet. Also available from Amazon: The Barnet Book Of Photography.
EVERYONE who has exposed a plate, and a large proportion of those who have not, are aware that the brightest colours in the scene that is being photographed are not those that are brightest in the picture, in which violet and blue appear brighter in comparison than green, yellow, and red. The photographer who has studied the matter knows that the ordinary plate, no matter how rapid it may be, has but little sensitiveness to green and still less to yellow, orange, and red. Although it is customary to employ ruby light in the dark room, it is quite possible with a little care to develop ordinary plates in yellow light, or in a shade of green that to the uninitiated viewing it for the first time resembles weak daylight.
It has long been the aim of plate-makers to produce a plate that shall be equally sensitive to all colours, and although this has not yet been accomplished, there are plates now obtainable that have a very considerable sensitiveness to all the colours of the spectrum. It is true that they are still more sensitive to violet and blue than to the other colours, but if we can contrive to lower the brilliance of the violet and the blue we shall obtain photographs in which every colour shall be represented in monochrome with exactly the shade of brilliance it has in nature. This sensitiveness to colour is obtained by the use of dyes, of which there are a large number, each of which has a different effect from the others, so that we may buy plates that are most suitable for landscape purposes, and others that are better fitted for subjects in which there is a preponderance of red.
For landscapes, portraits, and the work in which the amateur (and for the matter of that the professional worker), is most generally interested a plate that is sensitive to violet, blue, green, and yellow, leaves little to be desired, and is convenient to use, as it is possible to work it in plenty of light, so long as it is of the right quality. Such a plate is the " Barnet Yellow Sensitive Ortho-chromatic Plate," and to that I shall chiefly restrict myself in the present instance, as it is an exceedingly rapid plate (quite equal in speed to the Barnet Extra Rapid) and capable of doing all that is required in the ordinary way. So far as development is concerned there is nothing new to be learned. In loading the dark slides, in backing the plates as a preventive against halation, and during development, a little additional care to shield them from the light is necessary, but if the dark-room lamp is of a good type no alteration is needed.
As the formula for the developer is set forth in full on the box, it is unnecessary to give it in detail here, but for the benefit of those who work the 10 per cent, system, I may say that each working ounce contains approximately 3 grains of pyro, I grain of potassium meta-bisulphite, \ grain of potassium bromide, 22 grains of sodium carbonate, and 27^ grains of sodium sulphite. Other developers may, of course, be used if preferred, with the exception of hydro-ammonia and hydroquinone, which are not recommended. Beginners to whom the general manipulation of the plate is a matter of uncertainty are referred to other sections of this book.
It may be pointed out that an orthochromatic plate will do all that any other plate of the same speed will do, and in addition will do a great many things that the others will not do, so the photographer need not hesitate to load his slides or changing-box with them. He may, if he chooses, use them in precisely the same way as he would ordinary plates, or he may employ the special contrivances to be noted later on, which will enable him to get the full advantage out of them.
We all know that white sunlight is composed of various colours, and that the spectrum is arbitrarily divided into seven tints—violet, indigo, blue, green, yellow, orange, and red. We also know that every colour we see is either one of these colours, or is composed of two or more of them. As a matter of fact, every colour we see in nature reflects to our eyes, and incidentally to the lens of the camera, a proportion of white light in addition to its own particular colour. If this were not so we should be unable with an exposure of reasonable length on an ordinary plate to get detail in green, yellow, orange, and red subjects. As it is, the white light which these objects reflect is sufficient to give representations of them, though somewhat imperfectly.
As already stated, the orthochromatic plate is sensitive to all the colours, though chiefly to the violet and the blue (indigo as a purely arbitrary term is generally considered in photographic matters to be included in these two), and if we were to give a sufficient exposure to allow the other colours time to impress themselves on the plate, these first would be hopelessly over-exposed. If, however, we interpose somewhere in front of the plate a screen which shall reduce in brilliancy the most powerfully-acting light rays we shall give the greens, yellows, and oranges an opportunity of asserting themselves. As the plate at present under consideration is not specially sensitive to red we may leave it out of account.
The media used for screens are of various kinds: stained glass, glass coated with gelatine or collodion, and glass troughs filled with dyed solutions are all used. The last are useful in laboratory and experimental work, as the strength and also the tint of the dye may be easily altered, but in addition to being very expensive they are troublesome to carry about, and are not suitable for field and studio work. Stained glass would be the best of all if it were possible to get it of the correct colour and of the required strength. Many glass screens are sold and used, but I have found none that will correct the colours perfectly, so we are left with glass coated with some dyed material. These may be bought or they may be made by the photographer with comparatively little trouble. I have said that the different dyes used in preparing the plates give widely different degrees of colour sensitiveness, and it follows that a screen that is perfect with one maker's plate may be quite unsuitable for use with another, so when perfect correction is required and the maker of the plate does not issue a screen it is better to make screens for ourselves.
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