3. A table for facilitating reference to topics, names, and the like, in a book; -- usually alphabetical in arrangement, and printed at the end of the volume. 4. A prologue indicating what follows. [Obs.] --Shak. 5. (Anat.) The second digit, that next pollex, in the manus, or hand; the forefinger; index finger. 6. (Math.) The figure or letter which shows the power or root of a quantity; the exponent. [In this sense the plural is always indices.] Index error, the error in the reading of a mathematical instrument arising from the zero of the index not being in complete adjustment with that of the limb, or with its theoretically perfect position in the instrument; a correction to be applied to the instrument readings equal to the error of the zero adjustment. Index expurgatorius. [L.] See Index prohibitorius (below). Index finger. See Index, 5. Index glass, the mirror on the index of a quadrant, sextant, etc. Index hand, the pointer or hand of a clock, watch, or other registering machine; a hand that points to something. Index of a logarithm (Math.), the integral part of the logarithm, and always one less than the number of integral figures in the given number. It is also called the characteristic. Index of refraction, or Refractive index (Opt.), the number which expresses the ratio of the sine of the angle of incidence to the sine of the angle of refraction. Thus the index of refraction for sulphur is 2, because, when light passes out of air into sulphur, the sine of the angle of incidence is double the sine of the angle of refraction. Index plate, a graduated circular plate, or one with circular rows of holes differently spaced; used in machines for graduating circles, cutting gear teeth, etc. Index prohibitorius [L.], or Prohibitory index (R. C. Ch.), a catalogue of books which are forbidden by the church to be read; theIndex expurgatorius : [L.], or expurgatory index, is a catalogue of books from which passages marked as against faith or morals must be removed before Catholics can read them. These catalogues are published with additions, from time to time, by the Congregation of the Index, composed of cardinals, theologians, etc., under the sanction of the pope. --Hook. Index rerum [L.], a tabulated and alphabetized notebook, for systematic preservation of items, quotations, etc.