Page:Proceedings of the Royal Society of London Vol 69.djvu/42

34 ordinary commercial printing to control the extent to which they overlap; that is to say, the successive impressions will, owing to minute differences in " registering " the paper, have the dots of the different colours more or less displaced in relation to each other.

If, for instance, the blue dots and the pink dots happened each to exactly cover half the area, in one impression they might be exactly superimposed, and in another they might hardly overlap at all. In the former case half the area will be printed with both pink and blue, and half will be white. In the latter case half the area is printed with blue and half with pink, and there is no white left. If these impressions are to be equally good, the resulting absorptions through the spectrum .should be the same in the two cases.

Let the adjoining curves represent the percentage transparency of the inks. At A both inks are perfectly transparent, and as the colour is not absorbed by either, it can of course make no difference' to this colour if the dots are superimposed or not.

At B each colour absorbs half the light and reflects half the light. When the dots are printed adjacent to one another, half the total light of that colour will be reflected. When they coincide, half the paper is

left white : this reflects all the light it receives, which is half the total. The blue ink on the other half absorbs half the light which it