ectric vibration which is
at right angles to the direction of propagation.

3rd. There is the direction of the magnetic vibration or
motion which is at right angles to both of the other two.

Now we have seen that the direction of propagation of any aetherial
light ray, is that of a straight line from the sun corresponding to the
radius vector (Art. 76). We have also seen that the front of a light
wave is really that of a spherical shell (Art. 71).

We have also learned that the electric and the magnetic vibrations are
in the wave front, so that these two vibrations, which are at right
angles to each other, are to be found on the surface, so to speak, of
each aetherial spherical shell, that surrounds the sun with
ever-decreasing density, and ever-decreasing elasticity.

Let us try to picture the actual fact by an illustration. Let _S_ be the
sun, with concentric spherical aetherial shells surrounding it (Fig.
22). Then _S_ _A_ and _S_ _C_ will be rays of light being radiated out
from the sun, and the magnetic and electric vibrations have to be both
at right angles to the line of propagation and in the wave front; the
wave front being represented by the circular lines showing the section
of the concentric shells running north and south.

Now how can we picture these two motions at right angles to each other,
and yet both at right angles to the line of propagation? First, let us
take three straight lines and see how this may be done (Fig. 23).

Let _A_ _B_, _A_ _S_ be two straight lines at right angles to each
other, and _A_ _C_ another straight line at right angles to both. This
can only be done by making _A_ _C_ perpendicular to the plane of the
paper, and can be illustrated by supposing that it represents a pencil
or pen placed upright on the paper, the point of the pencil being at
point _A_. If this be done, then not only will _A_ _B_ and _A_ _C_ be at
right angles to each other, but both will be at right angles to _A_ _S_,
which corresponds to the line of propagation.

[Illustration: Fig: 23.]

Now refer to Fig. 22, and we shall see that the line _A_ _B_ and the
boundary of the shell will practically correspond. So that any section
of a spherical wave front will always be at right angles to the ray of
light. But we have learned from Art. 89 that these sections of the
aetherial spherical shell are really identical with Faraday's Lines of
Force, with the result that along any line which st