Indices of Refraction of Various Sub stances.— Any one definite chemical com pound has, at a given temperature, always the same index of refraction for the same ray. But glass is not such a compound, because its in gredients differ in different specimens of glass. It is impossible to make two pots of glass so exactly alike that they shall have the same index of refraction. It is owing to this fact that the idea frequently entertained, of making a telescope by putting together different pieces of glass, is impracticable. The following table shows the indices for a number of common transparent substances, and for three of the principal rays, C, D, F. It will be seen that glass has a greater refracting power than water ; and that plate glass is more refracting than crown. The diamond is the substance which has the greatest refracting power of all. It has also a great dispersive power; and it is owing to this fact that a diamond with polished faces shows so many brilliant colors when a ray of light falls upon it.
The refractive power of a substance is dif ferent at different temperatures. As a general rule, glass refracts more the colder it is. Con sequently the focal length of a telescope will generally be slightly shorter the colder the weather. The difference is, however, slight and there are exceptions to the rule.
Refraction by the It will be seen by the preceding table that air has an ap preciable, though slight, refracting pus; Cr, It follows that •a ray of light coming from a star is gradually bent toward the perpendicular as it approaches the surface of the earth. Only when a star is in the zenith does the ray suffer no refraction. The- result is that astronomical observations upon the declination or altitude of a heavenly body require to be corrected for refraction. The latter, depending upon the density of the air, is greater the colder the air and the higher the barometer. Hence the as tronomer has always to record the readings of his thermometer and barometer when he ob serves the altitude of a heavenly body. A
rough approximation to the amount of the re fraction can be made by the rule that at small zenith distances it is nearly 1" for each degree of zenith distance; and that, if the altitude is not greater than 45°, it increases nearly as the tangent of the zenith distance. The rate of in crease is somewhat less than that given by this law, but still the refraction increases rapidly as we approach the horizon, where it usually amounts to about 35'. This is greater than the diameter of the sun or moon. It follows that, when we see the lower limb of the sun just touching the horizon at sea, the luminary is actually entirely below the horizon. Moreover, the lower limb is elevated more by refraction than the upper one, with the result that a cer tain ellipticity is produced in the apparent out line of the sun, which is plainly perceptible to the naked eye.
A ray of light always suffers refraction when through the air in any other than a vertical direction, and its course is most curved when it is horizontal: In this case the curvature under ordinary conditions of the air is about one-sixth that of the earth. But this re sult depends entirely at the rate of increase or diminution of the temperature as we rise above the ground. If the air is very cold near the grouild, and rises rapidly in temperature above it, a horizontal ray may be more curved than the earth, and strike the latter at some point. Objects may then he seen at such a distance that they are far below the horizon, as was once noticed by an Arctic navigator who, when separated from one of his ships, saw her image upside down above the horizon and, sailing to ward it, found the ship. In the contrary case, when the air near the ground is much warmer than at a height above it, the reverse effect is produced. A horizontal ray is then refracted upward so as not to strike the earth at all. It is through this action that the mirages, so fre quent in the desert and elsewhere, are produced. See MIRAGE.