Let us assume that we have a painting or a drawing in colors from which it is desired to produce a set of printing plates to produce that drawing in facsimile. Under the old method of procedure, lithography, it would have been necessary to make a stone for each of the colors, which would mean, roughly speaking, from twelve to eighteen stones to reproduce it--it will be understood that this means the finished print must go through the press once for each color. This would mean twelve to eighteen impressions to get the desired result. The expense of doing this limited the use of lithography. [Illustration: HIGHLIGHT ENGRAVING] The modern or photo-engraving method of reproducing a colored copy is based on the theory of the three primary colors, yellow, red and blue. It is assumed that every color is formed by some combination of these three colors--the problem confronting us, therefore, is to separate these three colors and if possible make a printing plate of each color with the color values varying from light to dark in such proportions that when the three are printed in proper register over each other, with transparent printing inks, the varying color values will blend so as to reproduce the original. We go about this by making three negatives, one of each color, the red negative is made by placing at the lens a so-called color filter, which separates the red rays, whether they appear as pure red or any part of an orange or a purple, or any of the many tones of which red may form a part. In like manner the yellow and blue plates are made by the use of appropriate color filters, each of which acts for its required color as that used for the red. So far this would appear to be a purely mechanical operation, requiring simply the usual care in negative making, but unfortunately this theory does not work out so absolutely in practice, and for this reason, while any color may be produced in light rays by the union of the three primary colors of the proper quality, when the operation is attempted with material pigments or ink, produce results varying widely from the ideal. No pigment is absolutely pure, the adulterants or foreign substances will cause sufficient deviation from the abstract standard to cause a very noticeable difference in the finished result when united with another color which is of itself impure. The result is that the three negatives, instead of each being a true unit, ready for combination with the others, is really only a basis for further work. It might justly be compared with a sketch which is all right as far as it goes, but which requires toning down and elaboration before becoming a finished work of art.