bear very high magnification. If the eyepiece is now drawn toward the
observer, the star disk begins to expand; and if the mirror be a truly
spherical one, the expanded disk will be equally illuminated, except the
outer edge, which usually shows two or more light and dark rings, due to
diffraction, as already explained.

[Illustration: FIG. 8.]

Now if we push the eyepiece toward the mirror the same distance on the
opposite side of the true focal plane, precisely the same appearance will
be noted in the expanded star disk. If we now place our plane surface any
where in the path of the rays from the great mirror, we should have
identically the same phenomena repeated. Of course it is presumed, and is
necessary, that the plane mirror shall be much less in area than the
spherical mirror, else the beam of light from the artificial star will be
shut off, yet I may here say that any one part of a truly spherical mirror
will act just as well as the whole surface, there being of course a loss
of light according to the area of the mirror shut off.

This principle is illustrated in Fig. 3, where _a_ is the spherical
mirror, _b_ the source of light, _c_ the eyepiece as used when the plane
is not interposed, _d_ the plane introduced into the path at an angle of
45° to the central beam, and _e_ the position of eyepiece when used the
with the plane. When the plane is not in the way, the converging beam goes
back to the eyepiece, _c_. When the plane, _d_, is introduced, the beam is
turned at a right angle, and if it is a perfect surface, not only does the
focal plane remain exactly of the same length, but the expanded star
disks, are similar on either side of the focal plane.

[Illustration: FIG. 9.]