The property of reflection is also illustrated on a grand scale in the illumination of our satellite, the moon, and the various planetary bodies which shine by light reflected from the sun, and have no inherent self-luminosity. Aristotle was well aware that it is the reflection of light from the atmosphere which prevents total darkness after the sun sets, and in places where the sun's rays do not actually fall during the daytime. He was also of opinion that rainbows, halos, and mock suns, were all occasioned by the reflection of the sunbeams in different circumstances, by which an imperfect image of the sun was produced, the colour only being exhibited, but not the proper figure. The image, Aristotle says, is not single, as in a mirror, for each drop of rain is too small to reflect a visible image, but the conjunction of all the images is visible. Aristotle ascribed all these effects to the _reflection_ of light, and it will be noticed when we come to the consideration of the refraction of light, that of course his views must be seriously modified. [Page 272] The reflection of light is affected rather by the condition of the surface than the whole body of a substance, as a piece of coal may be covered with gold or silver leaf and caused to shine, whilst the brightest mirror is dimmed by the thinnest film of moisture. From whatever surface light is reflected, it always takes place in obedience to two fixed laws. First. _The incident and reflected rays always lie in the same plane._ Second. _The angle of incidence is equal to the angle of reflection._ With a single jointed two-foot rule, both of these laws are easily illustrated. The rule may be held in the hand, and one end being marked with a piece of white paper may be called the incident ray, _i.e._, the ray that falls upon the surface; and the other is the reflected ray, the one cast off or thrown back. A perpendicular is raised by holding a stick upright at the joint. (Fig. 260.) [Illustration: Fig. 260. A D. A two foot rule; the end A may be termed the incident ray, and the end D the reflected ray. S. The stick held perpendicularly. The angle A B C is equal to the angle D E F, and the whole may be moved in any direction or plane, either horizontal or perpendicular, G G. The reflecting surface.]