The speeial utility of the half-tone block is in this, that it eau be printed with type. In order that this may he done. the metal plate is backed np by a block of wood of the right thickness, so that the surfaces of the prepared metal shall be exactly even with the front of the types. With reasonably careful work in the preparation of the stereotype plates and the like, a page may he printed of typography with pictures of full light and shade incrusted in it the whole printed to gether, and that even on the power press.
The line-Cut Proeess gives an exact facsimile of a drawing in solid or dotted black lines. Any drawing which an artist will make with a point, as of a pen, or a brush, and which does not in clude tints or half-tones, eon he reproduced in ex act faesimile. The process is similar to those described above. The light passing through the negative hardens the prepared surface, and the unsolicited stuff is washed away. In fact, this may be considered the original proeess from which the half tone process has been produced by natural evolut bin, that is by the development along obvious line-. The line-cut block can be used in connection with type exactly as half tones at a Ivry much less cost and with greater .facility less care in the manipulation. .facility l'atxTING Puo-rcc•Exce.\vin rAcEs. The most important process of this kind is that known as the Three-color Process. This has beef une a very familiar and widely used proc and promises well. The system is based upon the scientific fact that all the colors of the spec trum may be obtained from the three primaries, red, blue, and green. The rays of light are separated by Mean'. of what are vaned photo graphic filters, which are partly transparent sub stances which allow partisular ray- to pass while they step or restrain others. Thus one of the three filters will allow only blue rays to pass. Iiy this appliance used in connection with the half-tone process there may be produced a perfect blue picture, and again with the use of other filters a green and a red, or approxi mately red picture. The printing of all these,
one upon another, approximates very closely the complete chromatic result desired, that is to say, a picture in full color. The chief difficulties of the process are in the making of correct nega tives, accurate registration in printing, and ap propriately colored inks. The result so far may be said to approximate to the reproduction of fine manual painting. One obvious reason for this imperfection of the result is found in the in ferior effect of colors superimposed to those which are laid side by side. Every student of painting knows how much more brilliant a gradation can be procured by putting touches of color close together, side by side, than by laying those colors over one another. It is true that much work, especially in oil painting. is done by su perimposed color, but this is when the upper coat of pigment is more or less translucent, as when the yellow below shows through the thin glaze of blue to produce a certain quality of green. Commonly the juxtaposition of color is more bril liant than the superimposition of color; and this fact causes an indefinitely great amount of thought and experiment among those who are trying to improve the three-eolor process.
Another and more complex process of photo graphic color work has been introduced in the United Mates within the last four yefirs, and some of the results are surprisingly atOurate reproductions of the originals. The greatest suc cesses in the color work have been obtained in the reproduction of colored textiles, notably in imitating the richness of color and texture of Oriental rugs. This is it secret process, and the methods employed are very carefully guarded. It is something more than the ordinary three-color work„ and apparently as many as seven colors are employed in some eases. It is as yet costly.