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Webster's 1913 Dictionary
\Un"du*la*to*ry\ (?; 277), a. [Cf. F. ondulatoire.]
Moving in the manner of undulations, or waves; resembling the
motion of waves, which successively rise or swell rise or
swell and fall; pertaining to a propagated alternating
motion, similar to that of waves.

{Undulatory theory}, or {Wave theory} (of light) (Opt.), that
   theory which regards its various phenomena as due to
   undulations in an ethereal medium, propagated from the
   radiant with immense, but measurable, velocities, and
   producing different impressions on the retina according to
   their amplitude and frequency, the sensation of brightness
   depending on the former, that of color on the latter. The
   undulations are supposed to take place, not in the
   direction of propagation, as in the air waves constituting
   sound, but transversely, and the various phenomena of
   refraction, polarization, interference, etc., are
   attributable to the different affections of these
   undulations in different circumstances of propagation. It
   is computed that the frequency of the undulations
   corresponding to the several colors of the spectrum ranges
   from 458 millions of millions per second for the extreme
   red ray, to 727 millions of millions for the extreme
   violet, and their lengths for the same colors, from the
   thirty-eight thousandth to the sixty thousandth part of an
   inch. The theory of ethereal undulations is applicable not
   only to the phenomena of light, but also to those of heat.