874. Practical Three-colour Selection. At a time when panchromatic emulsions were not available, the three negatives for three-colour selection were taken on an ordinary plate, on a plate sensitive to green and on a plate sensitive to red. Selection is now almost always done on panchromatic material of the same batch of emulsion, and except where the use of flexible film cannot be avoided, glass plates are used so as to avoid any risk of shrinkage of the support.
When daylight is not available or is too weak or too variable, the artificial light usually employed is that supplied by high-power in candescent lamps, which favours exposure under the red filter and balances the individual exposures and, as a rule, reduces the total exposure.
In order to check the perfect balance between the three images at all stages of the work, it is necessary to add to the object a scale of neutral grey tones produced photographically ; each of the areas must be of sufficient size to permit their densities to be measured on the negative ; this scale must be placed in such a position that its image can he cut away after the work is done. A photograph in colour is perfect only if it reproduces correctly a scale of neutral-grey tones, and this is possible only if the images of these greys have the same strength in the three negatives.
To facilitate registration, and the correct superposition of the three elementary positives, two marks are placed at the edges of the images. These marks must be as far from each other as possible. These guide-marks are crosses drawn on Bristol board, usually white on a black ground, the lines being sufficiently thick for their image to be visible after reduction in size. As a rule, several crosses are intermingled, the lines being of different thicknesses, and the mark chosen is that with the finest lines still clearly visible.
Finally, in order to identify easily the three negatives, it is usual to photograph not only the grey scale and the two marks, but also a test piece consisting of patches of red, green, and blue-violet, or greenish-blue, pink, and yellow, each patch bearing in black letters its name (or at least the initial of its name).
Focussing must be done after one of the filters has been fitted to the lens. As a rule, the green filter is chosen because it gives the most luminous image,' and also because it is the green image which occupies the middle position when the three images are not exactly in the same plane.
The exact exposure is of greater importance in tricolour photography than in monochrome work, because corrections cannot be made without upsetting the balance of the three negatives. Use must therefore be made of some form of exposure indicator (§ 324).
Even when selection is effected with plates of the same batch of emulsion, development in identical conditions (simultaneous immersion for the same length of time in time same de veloper) usually gives the negative taken under the blue filter a contrast factor (gamma) in ferior to that of the two other negatives. It is, therefore, necessary to ascertain by sensito metric tests or by trial and error the equivalent durations of development for the three nega tives; I- as a general rule the yellow printer is developed for 20 to ioo per cent longer than the two others.
After drying, it is necessary to check the register. One of the surest means consists in printing a transparency from one of the nega tives. It is possible to use a narrow strip cut from a plate of sufficient length to permit of both the register marks being included ; film should never be used because of the risk of play in the support. After drying this transparency, it is placed on each of the three negatives in succession, when it can be seen whether all three images and the register marks are in coincidence.
The usual trouble caused by the retouching required by prints in colour produced by sub tractive synthesis, has been considerably re duced or even done away with by doubling one or several of the selection negatives, developed to a value y, above the desired value with a transparency copied under another negative and developed to For instance, the pink and yellow negatives are doubled with a positive of the blue-green (A. Murray, 1933, following a suggestion of E. Albert, 1897).
875. Special Apparatus for Tricolour Selection. While, in the case of inanimate objects, any photographic camera may be used that is sufficiently rigid not to be put out of adjustment during the changes of the filters and dark slides, required when making the three exposures, it is obviously impossible to photograph in this way animate objects, since such are bound to move during the period of the three exposures and the changes of apparatus. Even landscape photography can be attempted only in excep tionally calm weather, otherwise the tops of trees may not occupy the same positions in the three elementary images.