Sight is considered the most important of the sensations. It is the chief means of bringing the body into proper relations with its surroundings and, even more than the sensation of hearing, is an avenue for the reception of ideas. The sense organs for the production of sight are the eyes; the external stimulus is *Light.*—Light, like sound, consists of certain vibrating movements, or waves. They differ from sound waves, however, in form, velocity, and in method of origin and transmission. Light waves are able to pass through a vacuum, thus showing that they are not dependent upon air for their transmission. They are supposed to be transmitted by what the physicist calls ether—a highly elastic and exceedingly thin substance which fills all space and penetrates all matter. As a rule, light waves originate in bodies that are highly heated, being started by the vibrations of the minute particles of matter. Light is influenced in its movements by various conditions. In a substance of uniform density it moves with an unchanging velocity and in a straight line. If it enters a less dense, or rarer, substance, its velocity increases; if one more dense, its velocity diminishes; and if it enters either the rarer or denser substance in any direction other than perpendicularly, it is bent out of its course, or _refracted_. If it strikes against a body lying in its course, it may be thrown off (_reflected_), or it may enter the body and either be passed on through (_transmitted_) or _absorbed_ (Fig. 157). Light which is absorbed is transformed into heat. *Kinds of Reflection.*—Waves of light striking against the smooth surface of a mirror are thrown off in definite directions, depending on the angle at which they strike. (Illustrate by holding a mirror in the direct rays of the sun.) But light waves that strike rough surfaces are reflected in practically all directions and apparently without reference to the angle at which they strike. (Illustrate by placing a piece of white paper in the direct rays of the sun. It matters not from what direction it is viewed, waves of light strike the eye.) This kind of reflection is called _diffusion_, and it serves the important purpose of making objects visible. The light waves passing out in all directions from objects which have received light from the sun, or some other luminous body, enable them to be seen. [Fig. 157] Fig. 157—*Diagram illustrating passage of light waves.*On the right the light is transmitted by the glass, reflected by the mirror, refracted by the prism, and absorbed by the black cloth. On the left the light from the candle forms an image by passing through a small hole in a cardboard and falling upon a screen.