\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.