Another device more frequently used by astronomers for the production of images (condensation of energy) is a lens which in its simplest form is a round piece of glass, thick in the center and thin at the edge, with a cross section, such as is shown at _A B_ in Fig. 38. If we suppose _E G D_ to represent a small part of a wave front coming from a very distant source of radiant energy, such as a star, this wave front will be practically a plane surface represented by the straight line _E D_, but in passing through the lens this surface will become warped, since light travels slower in glass than in air, and the central part of the beam, _G_, in its onward motion will be retarded by the thick center of the lens, more than _E_ or _D_ will be retarded by the comparatively thin outer edges of _A B_. On the right of the lens the wave front therefore will be transformed into a curved surface whose exact character depends upon the shape of the lens and the kind of glass of which it is made. By properly choosing these the new wave front may be made a part of a sphere having its center at the point _F_ and the whole energy of the wave front, _E G D_, will then be condensed at _F_, because this point is equally distant from all parts of the warped wave front, and therefore is in a position to receive them simultaneously. The distance of _F_ from _A B_ is called the focal length of the lens, and _F_ itself is called the focus. The significance of this last word (Latin, _focus_ = fireplace) will become painfully apparent to the student if he will hold a common reading glass between his hand and the sun in such a way that the focus falls upon his hand. [Illustration: FIG. 38.--Illustrating the theory of lenses.] All the energy transmitted by the lens in the direction _G F_ is concentrated upon a very small area at _F_, and an image of the object--e. g., a star, from which the light came--is formed here. Other stars situated near the one in question will also send beams of light along slightly different directions to the lens, and these will be concentrated, each in its appropriate place, in the _focal plane_, _F H_, passed through the focus, _F_, perpendicular to the line, _F G_, and we shall find in this plane a picture of all the stars or other objects within the range of the lens. [Illustration: FIG. 39.--Essential parts of a reflecting telescope.] 78. TELESCOPES.--The simplest kind of telescope consists of a concave mirror to produce images, and a magnifying glass, called an _eyepiece_, through which to examine them; but for convenience' sake, so that the observer may not stand in his own light, a small mirror is frequently added to this combination, as at _H_ in Fig. 39, where the lines represent the directions along which the energy is propagated. By reflection from this mirror the focal plane and the images are shifted to _F_, where they may be examined from one side through the magnifying glass _E_.