Source : Webster's Revised Unabridged Dictionary (1913)

Spectrum \Spec"trum\, n.; pl. {Spectra}. [L. See {Specter}.]
   1. An apparition; a specter. [Obs.]

   2. (Opt.)
      (a) The several colored and other rays of which light is
          composed, separated by the refraction of a prism or
          other means, and observed or studied either as spread
          out on a screen, by direct vision, by photography, or
          otherwise. See Illust. of {Light}, and {Spectroscope}.
      (b) A luminous appearance, or an image seen after the eye
          has been exposed to an intense light or a strongly
          illuminated object. When the object is colored, the
          image appears of the complementary color, as a green
          image seen after viewing a red wafer lying on white
          paper. Called also {ocular spectrum}.

   {Absorption spectrum}, the spectrum of light which has passed
      through a medium capable of absorbing a portion of the
      rays. It is characterized by dark spaces, bands, or lines.
      

   {Chemical spectrum}, a spectrum of rays considered solely
      with reference to their chemical effects, as in
      photography. These, in the usual photogrophic methods,
      have their maximum influence at and beyond the violet
      rays, but are not limited to this region.

   {Chromatic spectrum}, the visible colored rays of the solar
      spectrum, exhibiting the seven principal colors in their
      order, and covering the central and larger portion of the
      space of the whole spectrum.

   {Continous spectrum}, a spectrum not broken by bands or
      lines, but having the colors shaded into each other
      continously, as that from an incandescent solid or liquid,
      or a gas under high pressure.

   {Diffraction spectrum}, a spectrum produced by diffraction,
      as by a grating.

   {Gaseous spectrum}, the spectrum of an incandesoent gas or
      vapor, under moderate, or especially under very low,
      pressure. It is characterized by bright bands or lines.

   {Normal spectrum}, a representation of a spectrum arranged
      upon conventional plan adopted as standard, especially a
      spectrum in which the colors are spaced proportionally to
      their wave lengths, as when formed by a diffraction
      grating.

   {Ocular spectrum}. See {Spectrum}, 2
      (b), above.

   {Prismatic spectrum}, a spectrum produced by means of a
      prism.

   {Solar spectrum}, the spectrum of solar light, especially as
      thrown upon a screen in a darkened room. It is
      characterized by numerous dark lines called Fraunhofer
      lines.

   {Spectrum analysis}, chemical analysis effected by comparison
      of the different relative positions and qualities of the
      fixed lines of spectra produced by flames in which
      different substances are burned or evaporated, each
      substance having its own characteristic system of lines.
      

   {Thermal spectrum}, a spectrum of rays considered solely with
      reference to their heating effect, especially of those
      rays which produce no luminous phenomena.

Diffraction \Dif*frac"tion\, n. [Cf. F. diffraction.] (Opt.)
   The deflection and decomposition of light in passing by the
   edges of opaque bodies or through narrow slits, causing the
   appearance of parallel bands or fringes of prismatic colors,
   as by the action of a grating of fine lines or bars.

         Remarked by Grimaldi (1665), and referred by him to a
         property of light which he called diffraction.
                                                  --Whewell.

   {Diffraction grating}. (Optics) See under {Grating}.

   {Diffraction spectrum}. (Optics) See under {Spectrum}.