4. Any pellucid or transparent body, as air, water, and glass, which admits the free passage of light, is called a _medium_. When rays, after having passed through one medium, are bent out of their original course by entering another of different density, they are said to be _refracted_; and when they strike against a surface, and are sent back from it, they are said to be _reflected_. 5. A _lens_ is glass ground in such a form as to collect or disperse the rays of light which pass through it. These are of different shapes; and they have, therefore, received different appellations. A _plano-convex_ lens has one side flat, and the other convex; a _plano-concave_ lens is flat on one side, and concave on the other; a _double convex_ lens is convex on both sides; a _double concave_ lens is concave on both sides; a _meniscus_ is convex on one side, and concave on the other. By the following cut, the lenses are exhibited in the order in which they have been mentioned. [Illustration] 6. An _incident ray_ is that which comes from any luminous body to a reflecting surface; and that which is sent back from a reflecting surface, is called a _reflected ray_. The _angle of incidence_ is the angle which is formed by the incident ray with a perpendicular to the reflecting surface; and the _angle of reflection_ is the angle formed by the same perpendicular and the reflected ray. 7. When the light proceeding from every point of an object placed before a lens is collected in corresponding points behind it, a perfect image of the object is there produced. The following cut is given by way of illustration. [Illustration] 8. The lens, _a_, may be supposed to be placed in the hole of a window-shutter of a darkened room, and the arrow at the right to be standing at some distance without. All the light reflected from the latter object towards the lens, passes through it, and concentrates, within the room, in a focal point, at which, if a sheet of paper, or any other plane of a similar color, is placed, the image of the object will be seen upon it. 9. This phenomenon is called the _camera obscura_, or dark chamber, because it is necessary to darken the room to exhibit it. The image at the focal point within the room is in an inverted position. The reason why it is thrown in this manner will be readily understood by observing the direction of the reflected rays, as they pass from the object through the lens. In the camera obscura, it is customary to place a small mirror immediately behind the lens, so as to throw all the light which enters, downwards upon a whitened table, where the picture may be conveniently contemplated. 10. From the preceding explanation of the camera obscura, the theory of vision may be readily comprehended, since the eye itself is a perfect instrument of this kind. A careful examination of the following representation of the eye will render the similarity obvious. The eye is supposed to be cut through the middle, from above downwards.