_All Impulses are propagated in a homogeneous elastic Medium with an
equable Velocity._
Every experiment relative to sound coincides with the observation
already quoted from NEWTON, that all undulations are propagated
through the air with equal velocity; and this is further confirmed
by calculations. (LAGRANGE. Misc. Taur. Vol. I. p. 91. Also, much
more concisely, in my Syllabus of a course of Lectures on Natural and
Experimental Philosophy, about to be published. Article 289.) If the
impulse be so great as materially to disturb the density of the medium,
it will be no longer homogeneous; but, as far as concerns our senses,
the quantity of motion may be considered as infinitely small. It is
surprising that EULER, although aware of the matter of fact, should
still have maintained, that the more frequent undulations are more
rapidly propagated. (Theor. mus. and Conject. phys.) It is possible,
that the actual velocity of the particles of the luminiferous ether may
bear a much less proportion to the velocity of the undulations than in
sound; for light may be excited by the motion of a body moving at the
rate of only one mile in the time that light moves a hundred millions.
_Scholium_ 1. It has been demonstrated, that in different mediums the
velocity varies in the subduplicate ratio of the force directly, and of
the density inversely. (Misc. Taur. Vol. I. p. 91. YOUNG’s Syllabus.
Art. 294.)
_Scholium_ 2. It is obvious, from the phenomena of elastic bodies
and of sounds, that the undulations may cross each other without
interruption. But there is no necessity that the various colours of
white light should intermix their undulations; for, supposing the
vibrations of the retina to continue but a five hundredth of a second
after their excitement, a million undulations of each of a million
colours may arrive in distinct succession within this interval of time,
and produce the same sensible effect, as if all the colours arrived
precisely at the same instant.