Refraction. — Besides the change of place, to which the heavenly bodies are subjected in consequence of the effects of parallax, atmospheric refraction gives rise to a considerable displacement; and it is this power which the air, in common with all transparent media, possesses, which renders a knowledge of the constitution of the atmosphere very important to the astronomer. “ 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 50 directions before it reached the eye, the star would nevertheless appear in a line described by the ray nearest the eye. The operation of this principle is seen when an oar, or any stick, is thrust into the 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.”