[Illustration: Fig. 15.--Light travels in Straight Lines.] But if there is no dust in the room, and the light appears on the floor or on the opposite wall, and a straight rod is held between the slit and the patch of light, or between both a thread is stretched, and a body is brought between the patch and the slit, the light becomes visible on the opaque body and disappears from the place at which it was visible. If then the opaque body is moved to and fro in the space indicated by the rod, the light always remains visible on the opaque body. It is, therefore, clear that the light proceeds along straight lines from the slit to the place where it is visible.” That light travels in straight lines is only true so long as it travels through air or through the same kind of substance. When a looking-glass is held in the path of the rays in a dusty room, it is seen that a beam of light starts from the looking-glass in a direction quite different from that of the original beam. This fact did not escape the notice of Alhazen, and he carefully observed the direction taken by the reflected beam in various circumstances. He had read the books written by the learned Greeks, and he knew from them that the reflected beam makes the same angle with the mirror as the original beam does. To this observation he added another, and a very important one. He found that when a flat surface, such as a piece of paper or cardboard, is held against the mirror so that it touches the original and the reflected beam, that surface or sheet is always upright on the mirror, and stands at right angles to its surface. This law, discovered by Alhazen, explains how we see trees reflected in a river or a pond pointing straight down, and not slanting to the right or to the left. If we had a big fiat surface which we could make to stand vertically on the water, that surface, if made to pass, through the tree, would also pass through its reflection in the water. [Illustration: Fig. 16--Effect of Refraction of Light by Water.] Most of us have observed that when a kitten sees its image in a looking-glass for the first time, it takes it for another kitten, but after a few attempts to make friends with the supposed companion it finds out that the image is only an illusion. This observation was explained by Alhazen, by pointing out that the eye always perceives objects in the direction in which the beam of light enters the eye. He investigated another curious illustration of this. When a coin is placed at the bottom of a dish, and we move away from the dish until the rim just hides the coin, the latter is, of course, invisible. But on filling the dish with water without changing our position, we find that the coin becomes visible again, being apparently raised, although it really remains at the bottom of the dish as before.