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The temperature and humidity conditions of this coating require to be adjusted with great precision, in order that the surface of the collodion may be of such exquisite flatness that it will receive the impression of these very fine lines when the coated green film is passed through a special type of rotary printing machine. The printing roller is an immensely heavy steel roller milled with 20 lines to the millimetre, and by an elaborate process of ink distribution it is coated evenly with the finest possible ink. This ink, I may say, has given us a great deal of work to do. You will know, of course, that the finest half-tone block used by printers has at most 200 lines to the inch. We are printing 1,000 lines to the inch, and instead of paper, we are printing on a difficult substance like photographic film. The inked roller touches the surface of the green film under great pressure and leaves on it a series of inked lines 1,000 to the inch. These lines are quite invisible without the use of a fairly powerful microscope. The printed film now passes through a bleaching bath, represented by the first bath in the diagram on the screen, where the space betw een the inked lines becomes instantly bleached. The whole function of the printed lines is to act as a protection, so that the bleaching bath cannot get to the green dye underneath it. Thisis what is meant in the literature by the term “‘ fatty resist.’
When the film leaves the bleaching bath it goes straight into a second bath containing highly concentrated red dye. The bleached spaces become inmediately dyed red. The film then travels on under a number of jets of water, which wash away the excess of red dye, and it then passes through a series of tanks containing a solvent of the printing ink. It is easy to see what happens. The solvent removes the printing ink, revealing once again the green lines which it protected, and you thus get a series of green
Bieaching. Dvtine. REMOVAL oF Greasy Lines.
Fig. 1 and red lines alternately such as you see in the photo-micrograph on the screen.
The next stage of the process is to print the second series of lines at right angles to these first ones. I might say that the first lines are printed at an angle of 67 degrees te the length of the film, so that the next series oflines, which provide the blue primary are printed at an angle of 23 degrees. Exactly the same process is followed, except that, in order to obtain the required balance of area between the blue, green and red patches, the width of the line is somewhat different.
Fig. 2
After the printing of the greasy lines the film is passed through a bleaching bath once more, then through a solution of the blueviolet dye, and is finally cleaned in the ink solvent. Theappearance of the film is now such as is shewn in the photo-micrograph (Fig. 3). In the case of talking pictures, we protect a track at the side of the film from receiving the resin, as seen in the next slide.
We have now a 1,000 ft. length of standard width film coated with the colour matrix or reseau. To coat the sensitive emulsion on top of this would be impossible, as the dyes have the well-knowneftect of acting as powerful desensitisers. The
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