Fig. 7.
The original grain plate contained, measured diagonally, five black points, five white interstices to the millimetre. The figures represent enlargements of a negative made with the above-named grain plate, and for this the grain plate was one millimetre distant from the sensitive plate.
The accompanying Figure 8, 1–6, represents very much enlarged the scheme of grain of an autotype from a perfectly-executed wash drawing, with a screen of five opaque and five transparent lines to the millimetre, in equal proportions of black and white, at a distance of one millimetre from the sensitive plate, with equal time of exposure before and after the turning of the screen, and obviously with correct exposure for the whole.
The scheme is only given here in six gradations of tone. As a matter of fact, a good autotype has at least from six to eight times the number of tones, as with a sharp screen and an original cleanly {72} and sharply drawn in many tones, the light reflects the least alteration of tone in the size of the grain.
Fig. 8.
As previously mentioned, the smallest points represent the highest lights of the drawing; the larger, according to our graphic representation, the darker tones, but a correct reproduction of the grain.
In a similar way obviously the formation of the lines on the negative behave with an interposed but not revolved screen. Here also the lines in the high lights of the negative appear more covered than in the shadows.
The proportion of grain can, if the original requires it, be so far altered by not giving equal exposures for each position of the screen—for instance, in the proportion of two to one or three to one. We obtain then on the given scheme less closed tone gradations, 3, 4, and 5, and just before 3, and between and after 5, tone gradations, no single black and white points, but more or less jagged black or white lines, whilst the open black grain in 1 and 2 becomes more an oval instead of the round form in 6, and beyond the round openings are drawn out into ovals.