will
be the first retarded by it, the other portions as they enter the
glass being retarded in succession. It is easy to see that this
retardation of the one end of the wave must cause it to swing round
and change its front, so that when the wave has fully entered the
glass its course is oblique to its original direction. According to
the undulatory theory, light is thus _refracted_.

With these considerations to guide us, let us follow the course of a
beam of monochromatic light through our glass prism. The velocity in
air is to its velocity in glass as 3: 2. Let A B C (fig. 25) be the
section of our prism, and _a_ _b_ the section of a plane wave
approaching it in the direction of the arrow. When it reaches _c_ _d_,
one end of the wave is on the point of entering the glass. Following
it still further, it is obvious that while the portion of the wave
still in the air passes over the distance _c_ _e_, the wave in the
glass will have passed over only two-thirds of this distance, or _d_
_f_. The line _e_ _f_ now marks the front of the wave. Immersed wholly
in the glass it pursues its way to _g_ _h_, where the end _g_ of the
wave is on the point of escaping into the air. During the time
required by the end _h_ of the wave to pass over the distance _h_ _k_
to the surface of the prism, the other end _g_, moving more rapidly,
will have reached the point _i_. The wave, therefore, has again
changed its front, so that after its emergence from the prism it will
pass on to _l_ _m_, and subsequently in the direction of the arrow.
The refraction of the beam is thus completely accounted for; and it
is, moreover, based upon actual experiment, which proves that the
ratio of the velocity of light in glass to its velocity in air is that
here mentioned. It is plain that if the change of velocity on entering
the glass were greater, the refraction also would be greater.

Sec. 4. _Double Refraction of Light explained by the Wave Theory_.

The two elements of rapidity of propagation, both of sound and light,
in any substance whatever, are _elasticity_ and _density_, the speed
increasing with the former and diminishing with the latter. The
enormous velocity of light in stellar space is attainable because the
ether is at the same time of infinitesimal density and of enormous
elasticity. Now the ether surrounds the atoms of all bodies, but it is
not independent of them. In ponderable matter it acts as if its
density were increased without a