Index of refraction

Index In"dex, n.; pl. E. {Indexes}, L. {Indices}(?). [L.: cf. F. index. See {Indicate}, {Diction}.] [1913 Webster] 1. That which points out; that which shows, indicates, manifests, or discloses; as, the increasing unemployment rate is an index of how much the economy has slowed. [1913 Webster +PJC]

Tastes are the indexes of the different qualities of plants. --Arbuthnot. [1913 Webster]

2. That which guides, points out, informs, or directs; a pointer or a hand that directs to anything, as the hand of a watch, a movable finger or other form of pointer on a gauge, scale, or other graduated instrument. In (printing), a sign [[hand]] (called also {fist}) used to direct particular attention to a note or paragraph. [1913 Webster]

3. A table for facilitating reference to topics, names, and the like, in a book, usually giving the page on which a particular word or topic may be found; -- usually alphabetical in arrangement, and printed at the end of the volume. Typically found only in non-fiction books. [1913 Webster +PJC]

4. A prologue indicating what follows. [Obs.] --Shak. [1913 Webster]

5. (Anat.) The second finger, that next to the pollex (thumb), in the manus, or hand; the forefinger; {index finger}. [1913 Webster]

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}.] [1913 Webster]

7. The ratio, or formula expressing the ratio, of one dimension of a thing to another dimension; as, the vertical index of the cranium. [Webster 1913 Suppl.]

8. A number providing a measure of some quantity derived by a formula, usually a form of averaging, from multiple quantities; -- used mostly in economics; as, the index of leading indicators; the index of industrial production; the consumer price index. See, for example, the {consumer price index}. [PJC]

9. (computers) A file containing a table with the addresses of data items, arranged for rapid and convenient search for the addresses. [PJC]

10. (computers) A number which serves as a label for a data item and also represents the address of a data item within a table or array. [PJC]

11. (R. C. Ch.), The {Index prohibitorius}, a catalogue of books which are forbidden by the church to be read; also called {Index of forbidden books} and {Index Librorum Prohibitorum}. [PJC]

{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; the index 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. [1913 Webster]