Photo-Lithography

by Georg Fritz

A further experiment was printing a design over the original, the strength of which had to be brought in correct proportion to the original; the impressing of a design was also tried, and when using this method the correct angle of the incident light when making the exposure had to be taken into account.

Further experiments, which were principally carried out by Mariot, Cronenberg, and others, were founded principally on the basis of breaking up the tones in printing. For this a lineature or screen on glass, or a gelatine film, was introduced between the negative and the sensitive film. Others, again, coated the blank glass plate with a lineature, and prepared the plate afterwards with collodion or gelatine emulsion for the exposure.

It will be thus seen how this method of reproduction developed step by step till net-work of silk muslin or woven horsehair, and finally grating images, strongly reduced by photography, were placed in front of the photographic plate.

Thus were efforts made to make half-tone pictures suitable for printing by litho- or typography, and although the end was very nearly attained, yet the crux was not quite solved. These methods did not, however, yield the desired result, namely, a beautiful sharp clear image. There was still required a considerable improvement of the existing methods, and this was made by Meisenbach, of Munich.

The principle by which Meisenbach prepared his images was essentially different from the previously-described results. He broke up the half-tones, also by means of a lineature, into a printable grain, but the process was essentially different from the previous methods, in that Meisenbach used a glass plate on which, on a black ground, a grating was drawn till the glass was laid bare, and in this way prepared a lineature which consisted of clear glass, transparent lines and absolutely opaque lines.

He produced, first, an ordinary negative, from this a positive, and from this, by the interposition of the lineature, the actual half-tone negative for making the printing plate. The lineature or screen was in the second exposure interposed before the sensitive plate, and half the time of exposure given; then the screen was turned till the line first obtained crossed the second at an angle of 90°, and then the exposure was completed. By this means an absolutely certain breaking up of the half-tones into mathematically exact points was attained, and the most important step made in making photography useful for preparing printing plates for the two principal methods, typographic and lithographic. Meisenbach has called his process “autotypy.” It was, indeed, {69} somewhat inconvenient, but had the advantage that on the negative and also on the positive any retouching that was necessary could be done.

I cannot here enter into the numerous simplifications and improvements of Meisenbach’s process which, as well in the photographic process as also in the preparation of the lineature, were in the course of time made partly by him and partly by Carl Angerer, Gillot, Bussod and Valadon, Lefmann, and others, since, on the one hand, it would carry us too far, and, on the other hand, they may be considered as a natural consequence of the discovery. I will only mention that it was found after a short time that the two exposures and the production of the necessary positive could be omitted, and that the lineature could be simply interposed before the sensitive plate in the first exposure, by which an important simplification of the work was attained, and, thanks to the continued perfecting of the processes, no detraction of the good results was thus produced.

I do not consider it superfluous to give a short explanation of the action of the interposed lineature on the photographic plate during exposure.

If we take, for the sake of simplicity, not a picture, but a scale with four or five-tone gradations from light to deep black, the light tones will act more or less on the sensitive photographic plate according to their degree of brightness, but as the rays of light have to pass through the cross-lined screen interposed between the lens and sensitive plate, and as the rays can only pass through the transparent parts and not through the opaque, no homogeneous surface is obtained on the negative, but a tone produced by the cross-lines of the screen. If the tone was very bright, the rays reflected with great intensity on the sensitive plate will completely decompose the silver film. Since they are prevented by the screen from acting with equal power on all places of the surface, these places will show very plainly on the negative, and the result is therefore a darker tone on the same, which is marked with bright fine lines, corresponding in thickness to the screen plate.

If the original tone was darker so many rays of light will not be reflected, and these have thus not the power to impress the screen on the sensitive plate in full intensity; the result is therefore on the negative a tone which does not contain such strong dark lines as the first; the tone appears lighter in the negative, and in the print from the same darker than the first.