Let us assume that we are photographing a red object, a flower-pot for instance. The red negative represents the pot by a dark deposit. The positive printed off will consequently show clear glass at that spot, the unaffected gelatine being soluble. So that to dye the plate would be to make all red _except_ the very part which we require red; and on holding it up to the light the flower-pot would appear as a white transparent patch. How then is the problem to be solved? Mr. Shepherd’s process is based upon an ordered system of prisoner-taking. Thus, as red in this particular case is wanted it will be attained by the _other two_ positives (which are placed in contact with the red positive, so that all three coincide exactly), robbing white light of all _but_ its red rays. Now if the other positives were dyed green and violet, what would happen? They would not produce red, but by robbing white light between them of red, green, and violet, would produce blackness, and we should be as far as ever from our object. The positives are therefore dyed, not in the same colours as the screens used when the negatives were made, but in their _complementary_ colours, _i.e._ as explained above, those colours which added to the colour of the screen would make white. The red screen negative is therefore dyed (violet + green) = blue. The green negative (red + violet) = pink. The violet negative (red + green) = yellow. To return to our flower-pot. The red-screen positive (dyed blue) is, as we saw, quite transparent where the pot should be. But behind the transparent gap are the pink and yellow positives. White light (= violet + green + red) passes through pink (= violet + red), and has to surrender all its green rays. The violet and red pass on and encounter yellow (= green + red), and violet falls a victim to green, leaving red unmolested. If the flower-pot had been white all three positives would have contained clear patches unaffected by the three dyes, and the white light would have been unobstructed. The gradations and mixtures of colours are obtained by two of the screens being influenced by the colour of the object. Thus, if it were crimson, both violet and red-screen negatives would be affected by the rays reflected by it, and the green screen negative not at all. Hence the pink positive would be pink, the yellow clear, and the blue clear. White light passing through is robbed by pink of green, leaving red + violet = crimson. COLOUR PRINTING.