f these envelopes, and as
these aetherial envelopes are themselves formed by aetherial atoms, the
wave is spread outwardly from any central point in a spherical form as
proved by experiment. Not only, therefore, is the vibration in the wave
front, but it is always transverse to the line of propagation, for the
simple reason that the surface of the spherical shell or envelope is
always at right angles to the radius vector or straight line which joins
any centre to the surface of a spherical envelope.

As soon as the aetherial atom which forms the spherical aetherial
envelope has reached the limit of its expansion, it seeks to recover its
former position because of its elasticity, with the result that the
whole envelope contracts again, and arrives at its original position in
space ready to accept motion again and transmit it onwards in the same
manner as before.

Thus, by the acceptance of an atomic and gravitating Aether, we may form
a physical conception of one of the greatest problems in optical
phenomena, viz. the transverse vibration of light which always takes
place in the wave front, and across the line of propagation. Whether
this explanation is exactly correct in detail, or not, I am convinced
that the true physical explanation of the problem is to be found in an
atomic and gravitating Aether, as hitherto a frictionless Aether has
failed even to suggest to any scientist how such a transverse vibration
can take place.

ART. 72. _Reflection and Refraction._--A ray or wave of light is said to
be reflected when it meets with an obstacle which opposes its free
passage and turns it back. We have illustrations of this law of
reflection in the case of water waves striking against a breakwater, or
a sound wave striking against the wall of a room. In either case the
wave is turned back, and reflection is the result. A ray or a wave of
light is said to be refracted when, in passing from one medium into
another, it is turned from the straight path in which it was going
before it entered the refracting medium. An illustration of the
refraction of light is to be found in the case of the glass lens, so
often used to converge the light waves into one focus. We have up to the
present dealt with only two theories of light, the Corpuscular theory
and the Undulatory or Wave theory. We have seen how both harmonize with
Huyghens' principle, and the question arises as to whether both can be
made to harmonize with the phenomena of