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33 Earth’s surface. Thus the ray of light passing from the star / is turned from its course as soon as it leaves the void space represented by the blue tint; so that, instead of passing on towards the Earth in the con- tinuous dotted line, it is refracted in such a manner as to fall upon its surface at the point d. Now by an observer at this point, it will be seen in the direction in which its rays last come to him; and this being in the line d g, the apparent place of the star will be at g, or above its real place. This is the case with all the heavenly bodies, except when in the zenith; at that point there is no change induced by refraction, since the rays fall perpendicularly upon the strata of the atmosphere; and any luminous body in that spot of the heavens is seen in its right place.—From the same cause it happens that we see the Sun and Moon before they really rise above the horizon, and after they set. For the horizon of an observer at d being represented by the line H O, when the Sun is at a, so as to be really below it, the rays passing off in the direction a b are prevented by the atmospheric refraction from going on to c, but are bent to d; and as they reach the eye of an observer in the line d e, the apparent place of the Sun will be at e, that is, above the horizon. The amount of refraction is greatest at the horizon, and gradually diminishes towards the zenith. The atmosphere contributes in another way to pro- long the duration of day-light; namely, by reflecting downwards some of those solar rays which penetrate its higher regions after they have ceased to fall directly upon the earth’s surface. Thus in Diagram XXYI., the rays h and i are reflected downwards in the direc- tion j, j, j, as they pass through different strata of the atmosphere, and thus indirectly illuminate a part of the earth to which the sun has set. It is in this man- ner that twilight is produced; the length of which is of course greater in temperate and arctic regions, where the sun sinks obliquely and gradually below the hori-