If we look at a print placed at a certain distance, the details of the engraver's work disappears, the dots and the shading are confounded with the white lines which separate them, and the eye perceives only a grayish tint more or less distinct; so also, if a red and blue powder be mingled together, the mixture gives us the impression of a violet colour, although every grain of each powder preserves its own proper colour. This is explained as follows. We have already stated that the internal surface of the retina presents a mosaic of extremely small terminal divisions, each one of which acts separately, and transmits to the brain one single impression at a time. If the image of a trace of the burin or of a grain of powder covers one of these divisions, the impression is single (see fig. 35, p. 156); but if two lines, one white and the other black, or two grains, one red and the other blue, are so small and so near together, that their images are in juxtaposition on the same division of the retina, the impression is mixed, and the brain perceives the sensation of gray or of violet. Or, in other words, in order that two minute luminous objects may be distinctly seen, the angle subtended upon the retina by their images must not be greater than the diameter of one of the retinal divisions. The distance of the two objects from the eye being determined, the measure of the angle subtended by them enables us to estimate the size of these divisions.