To obtain even illumination of the positives, a condenser is essential, and it is obvious that a projection lantern will considerably facilitate matters. One of the gratings is placed in contact with the sensitive film, the yellow glass interposed between the light and the condenser, and the image of the positive sharply focused on the film; then without disturbing its position at all, the grating should be removed and ink dots made on the film corresponding to the dots on the positive. These give one a visible means of registering the three images.
The grating with the lowest number of rulings is placed in contact with the film, the yellow glass removed and the exposure made through the positive taken through the red filter. The yellow safe-light must now be placed in front of the light, the positive taken through the green filter substituted for the red one, the next finer grating placed in contact with the film in place of the coarser one, the second exposure made, and the operation repeated for the third picture. A preferable procedure, however, is to make the third picture on a separate plate, using the finest grating, and turn the positive around so that the image is reversed, as this can then be used as a cover glass for the other two pictures. The exposed plates are washed in water at 35 ° C. (95 ° F.) and then stood up to dry. It should be noted that the ruling of the gratings should be vertical.
The results are without any color at all; but when properly viewed the colors are at once seen. The viewing arrangements are very simple, being nothing more than a double convex lens, against which is placed the grating-picture, and a metal disc with a small peephole in the center which is placed at the focus of the lens. The light should be practically a line and this can easily be arranged by using the edge of a bats-wing gas burner, or an electric light may be used with an opaque screen with a slit about one eighth inch wide cut in it. If the light, lens and peephole are all in line, one only sees the central white image but if the lens and peephole be placed at a slight angle, the colors instantly start into view. What this angular arrangement does, of course, is to throw the first order spectra into the eye. Knowing the colors of the original, one soon knows the particular angle that will give the correct coloring; but, as an experiment, the effect of different angles should be tried, for it is possible in this way to show green cherries or roses with red leaves, or blue roses with purple leaves.
In contact printing, the same care must be taken about registration, and it can easily be worked out in the same way by ink dots or card guides. The exposure is best made by direct sunlight, and the sensitive plate and the grating, with the positive slide outside, should be placed at the bottom of a narrow lidless box, painted black inside, and pointed to the sun. The exposure at midday will be about thirty seconds.
Having once obtained a successful result, any number can be printed from it by contact on bichromated gelatine. Failure of registration is shown by overlapping of the edges of objects in the result, and errors in exposure may be seen by the colors being incorrect. For instance, overexposure with the red grating will cause yellows to be too orange, and errors in the blue printing will make the greens too blue. Modifications of this process have been suggested, but as they complicate without material improvement, we can ignore them. T. Thorp used a single grating and changed its angle for each exposure. While this gives as good results, it necessitates the use of three illuminants with three separate pictures inclined with regard to the rulings, and the viewing apparatus is more complicated. H. E. Ives proposed to use a black and white line-screen as well as the positive, and change the angle of the grating after each exposure, but this necessitates rather careful mechanism for changing the angle and shifting the line-screen after each exposure.
As pointed out, as far as possible a line of light should be used for viewing, for if too broad a source is used one has numerous spectra superimposed and consequent lack of color, as white is thus formed. If the line of light is at right angles to the ruling of the gratings, only a narrow stripe of color is seen, as the spectra are formed on each side of the central beam parallel to it and to the direction of the rulings.