Alhazen proved that this is another case of illusion due to the bending of the beam of light. He showed this bending of the beam of light in a very ingenious manner. He took a big glass vessel, and filled it with water containing a few drops of milk. He took it into a dark room where he had already studied the reflection of light. He found that when the water in the glass vessel was placed in the beam of light, the beam appeared broken at the surface of the water, bending suddenly down more towards the floor, and making it shorter. The beam was made visible in the air by the suspended particles of dust, and in the water by the suspended particles of milk. Alhazen showed that the beam under water always makes a greater angle with the surface than does the original beam, but he did not succeed in finding out the exact relation between the two angles, nor was that relation discovered until several centuries afterwards. Like a true Arab, Alhazen was very fond of studying illusions of all kinds. But there was one appearance which was before his time not considered an illusion, but a reality. The sun and moon appear much larger when they are rising or setting than they do when they are high up in the sky. Alhazen, however, showed that the apparent size is in reality the same in both cases. He held a coin at arm’s length so that it just covered the rising moon. He again held the same coin at the same distance from his eye between himself and the moon when she was up in the sky. He found that the coin again just covered the moon, but that if the moon were seen reflected in a distant mirror so that it was apparently on the horizon it again appeared larger. This, he said, was simply owing to our habit of judging things on the earth by comparison with earthly objects. The Arabs, whether in Bagdad, Damascus, Cairo, or Cordova, were well acquainted with the principle of Archimedes, and they knew that a body in water weighs less than in air, the difference being the weight of the water displaced by the body. But they were first to use the idea of what is now known as specific gravity, and they explained that the specific gravity of a body is obtained by dividing the weight of the body by the weight of the same volume of water. This definition is used even now, and tables of specific gravity are found in all books dealing with the properties of various materials.