In order to understand the action of a lens, it is necessary to understand the action of a prism. By the aid of Fig. 20 the action of the lenses of which telescopes are constructed will be understood. A prism is a piece of glass, or other transparent substance, the sides of which are so inclined to each other that its section is a triangle, and its action on light passing through it is to change the direction of the course of the beam. If we examine Fig. 21 we shall understand the action clearly. It is a known law, that when a beam of light falls obliquely on the surface of a medium more dense than that through which it has been passing, its direction is changed to a new one, nearer the line drawn at right angles to that surface, railed the normal. For instance, the ray S, I, falling on the prism at I, is bent into the course I, E, which is in a direction nearer to that of N, I, produced inside the prism. On emerging, the reverse takes place, and the ray is bent away from the normal E, N´, and takes the course E, R. In the second diagram, Fig. 21, the ray S, I, called the incident ray, coincides with the normal to the surface, so it is not refracted until it reaches the second surface, when it has its path changed to E, R, instead of taking its direct course shown by the dotted line. This bending of the ray is very plainly shown with an electric lamp and screen. If a trough with parallel sides be placed so as to intercept part of the light coming from the electric lamp, so that part shall pass through it and part above, we have the image of the hole in the diaphragm of the lantern on the screen unchanged. Now, if the trough be filled with water, no difference whatever is made in the position of the light on the screen, because the water, which is denser than the air, is contained in a trough with parallel sides; but by opening the sides like opening a book, or by interposing another trough with inclined sides, shaped like a =V=, that parallelism is destroyed, and then the light passing through it will be deflected upwards from its original course, and will fall higher on the screen; by opening the sides more and more, one is able to alter the direction of the light passing through the prism, which has been constructed by destroying the parallelism of the two sides. [Illustration: FIG. 21.—Deviation of Light in Passing at Various Incidences through Prisms of Various Angles. ] The refraction of light then depends upon the density of the substance through which it passes, on the angle of incidence of the ray, on the angle of the prism, and also on the colour of the light, about which we shall have something to say presently. Let us now pass from the prism to the lens; for having once grasped the idea of refraction there will be no difficulty in seeing what a lens really is.