from
The Collaborative International Dictionary of English v.0.48
Undulatory \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.
[1913 Webster]
{Undulatory theory}, or {Wave theory} (of light) (Opt.), that
theory which regards the various phenomena of light 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.
[1913 Webster]