In the reproduction of deep black surfaces finally no rays of light are reflected, therefore no rays of light can penetrate through the screen, and because actually here no chemical change of the light sensitive film takes place, we obtain a negative which is clear and transparent in these places, that is to say without any interruption, which gives in the print a full black tone. {70}
In this process there comes, however, to our aid a very important physical law, namely, the diffraction of light. A ray of light passing through a round hole or a slit and falling on to a black surface is represented not the same size as the slit, but more or less broadened the more the receiving surface is moved away, and in the middle the brightest light will be, and this gradually fades off into shadow towards the margin.
If we take now the opposite, and use instead of the hole or slit a black surface or a conglomerate of such in the form of points, squares, or other geometrical figures, and if these be placed before a screen illuminated with a bright light, these figures become somewhat smaller with this bright illumination, whilst with a weaker illumination they are represented in correct strength.
Where the light acts in its full intensity, or in excess, in a manner of speaking it is diffracted or bent over the figures, and proportionately makes them smaller.
This law does good service in autotypic work.
As we have seen, the rays of light must pass through the screen during exposure. This screen consists of equally thick opaque black and transparent white lines. Where the light acts with full intensity it will be more diffracted over the black lines. It makes the lines of the screen broader on the negative, and will, therefore, have more covered places, which in the after printing of the prints cannot be penetrated by the light, and thus represent the light places in the print covered with fine dots.
The darker the tone the less the rays of light reflected, and the less, therefore, can the light be diffracted. As the tones get darker the lines of the screen become of equal value on the negative, till finally, when at the opposite end of the screen, they totally disappear in the deep blacks.
A valuable quality of a correctly-prepared autotype is, therefore, that the covered lines or the covered grain appear of different strengths on the negative. In the high lights they are stronger, in the half-tones weaker, till finally in the black parts they have totally disappeared.
A further not less valuable property is that the grain in the highest lights is not sharply defined, does not appear as a square point, but is rounded by the diffraction of light, which gives a softer, more beautiful appearance and plasticity.
Whilst the light reflected from the brightest parts of the drawing acts with full intensity, and is diffracted over the figures standing in its path and makes the same smaller, and cuts off the sharp corners and gives more covered surfaces to the negative, as it decreases according to the more or less deep shadows of the drawing, and can only act now with less intensity through the open places of the screen, the result is that the points become larger, till finally in the deepest parts it is quite inactive, and the shadows begin to block. The action of the light rays of different {71} strengths reflected on to the negative in accordance with the brighter and darker tones of the drawing, taking into consideration the print, can be graphically represented as a pyramid of which the base represents the deepest shadows and the point the high lights. If we interpose a cross-lined screen, or more correctly called a grain plate, before the sensitive plate, we obtain, graphically considered, grain figures, actually of many more very different sizes, as shown in Fig. 7, 1–6, in which the smallest points represent the high lights of the drawing, and those becoming gradually bigger the gradually increasing shadows.