tudy
it. Now let us put the star out of focus by turning the focusing screw.
Suppose we turn it in such a way that the eyepiece moves slightly
outside the focus, or away from the object glass. Very beautiful
phenomena immediately begin to make their appearance. A slight motion
outward causes the little disk to expand perceptibly, and just as this
expansion commences, a bright-red point appears at the precise center of
the disk. But, the outward motion continuing, this red center
disappears, and is replaced by a blue center, which gradually expands
into a sort of flare over the middle of the disk. The disk itself has in
the mean time enlarged into a series of concentric bright rings,
graduated in luminosity with beautiful precision from center toward
circumference. The outermost ring is considerably brighter, however,
than it would be if the same gradation applied to it as applies to the
inner rings, and it is surrounded, moreover, on its outer edge by a
slight flare which tends to increase its apparent width. Next let us
return to the focus and then move the eyepiece gradually inside the
focal point or plane. Once more the star disk expands into a series of
circles, and, if we except the color phenomena noticed outside the
focus, these circles are precisely like those seen before in
arrangement, in size, and in brightness. If they were not the same, we
should pronounce the telescope to be imperfect. There is one other
difference, however, besides the absence of the blue central flare, and
that is a faint reddish edging around the outer ring when the expansion
inside the focus is not carried very far. Upon continuing to move the
eyepiece inside or outside the focus we observe that the system of rings
becomes larger, while the rings themselves rapidly increase in number,
becoming at the same time individually thinner and fainter.

[Illustration: THE STAR IMAGE.]

By studying the appearance of the star disk when in focus and of the
rings when out of focus on either side, an experienced eye can readily
detect any fault that a telescope may have. The amateur, of course, can
only learn to do this by considerable practice. Any glaring and serious
fault, however, will easily make itself manifest. Suppose, for example,
we observe that the image of a star instead of being perfectly round is
oblong, and that a similar defect appears in the form of the rings when
the eyepiece is put out of focus. We know at once that something is