In what has been said about the visual image we have supposed that the light was monochromatic, or homogeneous. Let us see what will happen if the light is polychromatic, say, for example, sunlight, and let a beam of sunlight be intercepted on a screen after passing through a double convex lens. It will be observed, as in figure 23, that the violet rays are brought to a focus nearest the lens, and the red farthest away, and circles of light will be seen on the screen; this wandering of the colored rays from a common focus is called chromatic aberration and depends on the dispersive properties of the material of which the lens is made. Here is a defect that can not be corrected by a stop, but as the refractive and dispersive properties of a substance do not vary together, it is possible to combine two substances, one with high refractive and low dispersive properties and the other with the reverse properties. If proper curves are given to them they will correct each other, thereby producing coincidence of the visible and chromatic foci. Such a combination gives an achromatic lens, which is usually composed of a double convex of crown glass cemented to a diverging meniscus of flint glass, as shown in section in figure 24. This combination is not absolutely achromatic, but sufficiently so for all general purposes. [Illustration: FIG. 23.] [Illustration: FIG. 24.] TELESCOPES. The telescope is an optical instrument based on an object glass or reflector to form a real image of a real and distant object, and of an ocular to magnify and view the image. Telescopes are classified as refracting or reflecting according as the object glass is a lens or a reflector. The object glass must be essentially convex if the telescope is a refractor, and if a reflector, the object mirror must be concave; the ocular may be either concave or convex. There are four types of refractive telescopes used for military purposes, viz: 1. The astronomical. 2. The terrestrial. 3. The galilean. 4. The prismatic. Figure 26 is a section of an astronomical telescope. The object glass (_D_) is a combination consisting of a double convex and a double concave lens cemented together with Canada balsam. The double concave lens is added to correct for chromatic aberration. The ocular (_E_) is a convex-concave lens.