In order to understand the nature of refraction, we must consider, that an object always appears in the direction in which the _last_ ray of light comes to the eye. If the light which comes from a star were bent into fifty directions before it reached the eye, the star would nevertheless appear in the line described by the ray nearest the eye. The operation of this principle is seen when an oar, or any stick, is thrust into water. As the rays of light by which the oar is seen have their direction changed as they pass out of water into air, the apparent direction in which the body is seen is changed in the same degree, giving it a bent appearance,--the part below the water having apparently a different direction from the part above. Thus, in Fig. 22, page 96, if S _a x_ be the oar, S _a b_ will be the bent appearance, as affected by refraction. The transparent substance through which any ray of light passes is called a _medium_. It is a general fact in optics, that, when light passes out of a rarer into a denser medium, as out of air into water, or out of space into air, it is turned _towards_ a perpendicular to the surface of the medium; and when it passes out of a denser into a rarer medium, as out of water into air, it is turned _from_ the perpendicular. In the above case, the light, passing out of space into air, is turned towards the radius of the earth, this being perpendicular to the surface of the atmosphere; and it is turned more and more towards that radius the nearer it approaches to the earth, because the density of the air rapidly increases near the earth. [Illustration Fig. 22.] Let us now conceive of the atmosphere as made up of a great number of parallel strata, as A A, B B, C C, and D D, increasing rapidly in density (as is known to be the fact) in approaching near to the surface of the earth. Let S be a star, from which a ray of light, S _a_, enters the atmosphere at _a_, where, being much turned towards the radius of the convex surface, it would change its direction into the line _a b_, and again into _b c_, and _c_ O, reaching the eye at O. Now, since an object always appears in the direction in which the light finally strikes the eye, the star would be seen in the direction O _c_, and, consequently, the star would apparently change its place, by refraction, from S to S´, being elevated out of its true position. Moreover, since, on account of the continual increase of density in descending through the atmosphere, the light would be continually turned out of its course more and more, it would therefore move, not in the polygon represented in the figure, but in a corresponding curve line, whose curvature is rapidly increased near the surface of the earth.