Just what do we mean by angle of light? We must remember that the rays of light coming from the sun, radiate or travel outward in straight lines, as the spokes of a wheel radiate from the hub; except that the spokes of light are not confined to a flat circle, but radiate in all directions from every part of the sun’s orb. But to return to the wheel. Let us suppose that it is a buggy’s wheel, and that the buggy is jacked up, so that we can turn the wheel easily. We will do so until one of the spokes is pointing straight down to the ground, and, to make sure that it is exactly vertical, we will suspend in front of it a string with a weight attached to its lower end. If the spoke follows exactly the direction of this plumb line, then we know that it is pointing down directly to the surface of the ground. We know, in fact, that the direction of the spoke is at right angles to the surface of the ground; or, which amounts to the same thing, we may say that the surface of the ground is at right angles to the direction of the spoke.