that the pigment particles have a certain degree of transparency so that light partly penetrates them, certain rays being then reflected and certain absorbed according to the hue. A blue pigment, for example, absorbs all constituent rays of white light except the blue and the hues which border on blue in the spectrum, it being impossible to procure pigments which are so pure that they do not let some other hues besides their own characteristic one pass through them. Similarly with yellow, it absorbs all the spectral rays save the yellow, the orange, and the green. Adding these two pigments together, we get every spectral ray absorbed except green, a certain amount of which both pigments have allowed to pass. In a similar way we can explain why blue and red give purple and why a mixture of all the spectral colors as pigments produces a dark gray of uncertain hue.
The above applies to a matt surface; when there is any trace of glaze there comes into play another factor which we must now consider, namely, surface reflection of some white light which has not penetrated the pigment particles at all and which therefore causes the color to be more or less unsaturated. It is by diminishing surface reflection of white light that the colors of a picture may be raised in saturation by subjecting it to alcohol vapor, which softens the medium and removes surface cracks. Reflection of white light also takes place at the surface of the pigment particles themselves and is greatly diminished when these are extremely small, hence the importance in the manufacture of pigments of thorough grinding. It is further minimized by suspending the pigments in oil, because this causes the light before it strikes the surface of the pigment particles to pass through a medium which is of approximately the same density as that of the particles themselves. This reduces the reflection, because the greater the difference of density between two media the greater the reflection of light at the interface between them.
The quickly vibrating (blue) rays of the spectrum tend to be reflected more readily than the slowly vibrating (red) rays, hence we often find that a substance is bluish by reflected light, whereas it is reddish when the light passes through it. It is indeed for this reason that during the day the sky looks blue, the light being reflected from the fine particles of dust and moisture which are constantly suspended in it, whereas after the sun has set it is red because the slanting rays come to be transmitted through these particles.
Artificial illumination alters the hues of pictures mainly because of mixture of colored lights, that is to say, of the hue of the light reflected from the surface of the picture and of the hue due to the particular pigments employed. Thus, if we regard a picture in yellow light (gas, carbon filament, etc.) the pale blues may appear white (mixing of complementary colors), the deeper blues assume a greenish hue, and the reds turn to orange.
