permeable by the water to facilitate its removal from the gelatine prior to development. To the quality and proportions of the color and gelatine we refer in another chapter. As the tissue is prepared and sent out ready for use by the patentee, it is not important to enter into more minute details of its preparation here.
The tissue is prepared in three distinct varieties of color; and in each scale there are three gradations of intensity, to suit negatives of various kinds. The colors are described as Indian-ink, sepia, and photographic purple.
The Indian-ink tissue is a pure black, nearly neutral in tone, but inclining to warmth rather than coldness.
The sepia tissue is of a rich deep brown, of a warm sepia tint.
The photographic purple tissue is of a tint resembling that common in gold-toned silver prints, generally of a purple-brown character, or in its extreme depths a purple-black.
Each of these tints is made in three qualities, to suit the different degrees of intensity in negatives, on a principle first pointed out by Mr. Swan, and which it may be well here to explain in detail.
In this method of pigment printing, although the best picture will result from the best negative, it is possible with a very intense hard negative, possessing very abrupt contrasts, to produce extremely soft and harmonious prints; whilst, on the other hand, brilliant prints may be obtained from a feeble negative possessing very little contrast or intensity. The principle upon which these effects are secured is this: The reader has seen that, as the gradations in the picture are obtained by different thicknesses of a translucent colored film resting on a white ground, the deepest shade being secured by the greatest thickness of this material, most completely covering up
