THE question often arises, What is pure black? Teachers of physics say that black is an absence 'of all color-that is, no matter what light falls on a pure black substance none will be reflected, all being The extreme of blackness. absorbed. There is no such substance known to man which will do this thing but, as usual, ingenuity has overcome this disadvantage. The standard of blackness and, than which none can be blacker, is simply a hole in a black box. The physical reasons for this are easy to see. The only condition necessary is that a minimum number of rays of light shall be reflected from the standard of blackness. To show the advantage of the hole we will contrast the rays reflected from the black face of the box with the number coming out of the hole. Let a ray beam of light strike the face and hole normally or obliquely. The face at once shows itself lighter than the hole for this reason: The rays striking the face are diffusely reflected in every imaginable direction and at every imaginable aDerle, consequently an eye in front of the face will receive a certain number of these diffusely reflected beams. But now consider the rays that go through the hole and that strike the back of the box (also painted black, and of a rough surface, as is the face). These will also be reflected in every imaginable direction and for that reaSOn the majority of them will not come out of the hcle but be caught on the inside of the face being either absorbed or reflected again. So if a fewer number of rays come out the appearance will be the more black to the eye, as the eye only “sees” when a ray of light enters it. Illustrations of this principle are common. If one looks into an open window of a house on a very bright day, even, the window looks simply like a black hole-the light getting in is not refected back again in near the amount of entering, even though the inside wall be of the same color as the outside wall.