The initial positions of the pair must be marked plainly, care must be taken not to displace the spacers 120° apart often found at the edges of the lenses, and the various positions must be tried in an orderly manner. One not infrequently finds a position in which the fault is negligible or disappears altogether, which point should be at once marked for reference. [Illustration: FIG. 159.—Correct Image Just Out of Focus.] In case there is uncorrected spherical aberration there is departure from regular gradation of brightness in the rings. If there is a “short edge,” _i.e._, + spherical aberration, so that rays from the outer zone come to a focus too short, the edge ring will look too strong within focus, and the inner rings relatively weak; with this appearance reversed outside focus. A “long edge” _i.e._, - spherical aberration, shows the opposite condition, edge rings too strong outside focus and too weak within. Both are rather common faults. The “long edge” effect is shown in Figs. 160 and 161, as taken quite close to focus. It takes a rather sharp eye and considerable experience to detect small amounts of spherical aberration; perhaps the best way of judging is in quickly passing from just inside to just outside focus and back again, using a yellow screen and watching very closely for variations in brightness. Truth to tell a small amount of residual aberration, like that of Fig. 160, is not a serious matter as regards actual performance—it hurts the telescopist’s feelings much more than the quality of his images. [Illustration: FIG. 160.—Spherical Aberration Just Inside Focus.] [Illustration: FIG. 161.—Spherical Aberration Just Outside Focus.] A much graver fault is zonal aberration, where some intermediate zone of objective or mirror comes to a focus too long or too short, generally damaging the definition rather seriously, depending on the amount of variation in focus of the faulty zone. A typical case is shown in Fig. 162 taken within focus. Here two zones are abnormally strong showing, just as in the case of simple spherical aberration, too short focus. Outside of focus the intensities would change places, the outer and midway zones and center being heavy, and the strong zones of Fig. 162 weak. These zonal aberrations are easily detected and are rather common both in objectives and mirrors, though rarely as conspicuous as in Fig. 162.