The object, whatever it is, should be set up in the sun against a dark background distant say 40 or 50 times the focal length of the objective to be tested. The writer rather likes a silvered ball cemented to a big sheet of black cardboard. At night a pin hole say 1/32 inch or less in diameter through cardboard or better, tinfoil, with a flame, or better a gas filled incandescent lamp behind it, answers well. The latter requires rather careful adjustment that the projected area of the closely coiled little filament may properly fill the pinhole just in front of it. Now if one sets up the telescope and focusses it approximately with a low power the star can be accurately centered in the field. Then if the eyepiece is removed, the tube racked in a bit, and the eye brought into the focus of the objective, one can inspect the objective for striæ. If these are absent the field will be uniformly bright all over. Not infrequently however one will see a field like Fig. 152 or Fig. 153. The former is the appearance of a 4 inch objective that the author recently got his eye upon. The latter shows typical striæ of the ordinary sort. An objective of glass as bad as shown in Fig. 152 gives no hope of astronomical usefulness, and should be relegated to the porch of a seashore cottage. Figure 153 may represent a condition practically harmless though undesirable. The next step is a really critical examination of the focal image. Using a moderately high power ocular, magnifying say 50 to the inch of aperture, the star should be brought to the sharpest focus possible and the image closely examined. If the objective is good and in adjustment this image should be a very small spot of light, perfectly round, softening very slightly in its brilliancy toward the edge, and surrounded by two or three thin, sharp, rings of light, exactly circular and with well defined dark spaces separating them. [Illustration: FIG. 152.—A Bad Case of Striæ.] [Illustration: FIG. 153.—Ordinary Striæ.] Often from the trembling of the air the rings will seem shaky and broken, but still well centered on the star-disc. The general appearance is that shown in Fig. 154.[29] [29] This and several of the subsequent figures are taken from quite the best account of testing objectives: “On the Adjustment and Testing of Telescope Objectives.” T. Cooke & Sons, York, 1891, a little brochure unhappily long since out of print. A new edition is just now, 1922, announced. [Illustration: FIG. 154.—A First Class Star Image.]