BEHAVIOUR OF WHITE LIGHT takes place when light is passing from a transparent substance of lower to one of higher refractive index. Though water is not of so high refractive index as glass, it is very much higher than air, and as we see the glass by scattered reflection, if the reflection no longer takes place, the glass disappears, just as in the story by Wells of the invisible man he became invisible by reducing the refractive index of all his tissues to the refractive index of air. We can at once proceed to make some very important practical applications of these experiments. We are ;
accustomed to regard some pigments as transparent or, at any rate, translucent, and others as opaque. It is a question which we shall have to discuss later whether there is any such thing as an opaque pigment. We shall, at any rate, find that many pigments usually regarded as opaque are transparent in a highly refractive medium. In the meantime we shall confine our researches to the bearing which our observations have on white lead and zinc white, so as to avoid the complications of colour which we shall have to consider presently. If white lead is ground in a liquid of highly refractive index like bromonaphthalene, it becomes a translucent pigment of a low-toned greyish white and if the ground pigment is flooded ^vith bromonaphthalene and examined under the microscope by transmitted light, it is seen to consist of transparent crystals with a high refractive index, gUttering hke diamonds. White lead, therefore, is an extreme case of our powdered glass, and the amount of white light it will reflect will depend on the refractive index of the liquid in which it is immersed. It reflects most light as a dry powder in air and reflects less light when ground in a medium like egg or size, and still less light when ground in oil, which has a higher refractive index than either egg or size. ;
108