Much of this work is well within the useful range of instruments of three to six inches aperture. The strategy of successful investigation is in turning attention upon those things which are within the scope of one’s equipment, and selecting those which give promise of yielding to a well directed attack. And to this end efforts correlated with those of others are earnestly to be advised. It is hard to say too much of the usefulness of directed energies like those of the Variable Star Association and similar bodies. They not only organize activities to an important common end, but strengthen the morale of the individual observer. INDEX A Abbé, roof prism, 162 Aberration, compensated by minute change of focus, 266 illuminates the diffraction minima, 265 relation determines of focus and aperture, 266 Achromatic long relief ocular, 146 objective, 77 Achromatism, condition for, 78 determination of, 78 imperfection of, 87 Adjustment where Polaris invisible, 235 Air waves, length of, 255 Alt-azimuth mount for reflector, 102 mounts, with slow motions, 102 setting up an, 228 Anastigmats, 84 Annealing, pattern of strain, 68 Astigmatism, 84, 209 of figure, 210 Astronomy, dawn of popular, 19 B Bacon, Roger, alleged description of telescopes, 6 Barlow lens, 152 “Bent,” objective, 86 Binocular, 2 advantage of, exaggerated, 151 for strictly astronomical use, 152 telescopes for astronomical use, 163 C Camouflage, in optical patents, 97 Cassegrain, design for reflecting telescope, 22 Cassegrain, sculptor and founder of statues, 22 Cell, taking off from a telescope, 202 Chromatic aberration, 11, 76 investigation of, 210 correction, differences in, 91 error of the eye, 90 Clairault’s condition, 81 two cemented forms for, 81 Clarks, portable equatorial mounting, 109 terrestrial prismatic eyepiece, 158 Clock, the cosmic, 233 Clock drive, 110, 174 Clock mechanism, regulating rate of motor, 179 Coddington lens, 137 Cœlostat constructions, 126 tower telescopes, 127 Color correction, commonly used, 211 examined by spectroscope, 211 of the great makers, 90 Coma-free, condition combined with Clairault’s, 83 Comet seeker, Caroline Herschel’s 118 seekers with triple objective, 119 Crowns distinguished from flints, 64 Curves, struggle for non-spherical, 18 D Davon micro-telescope, 148 Dawes’ Limit, 261 in physiological factors, 263 Declination circle, 108 adjustment of, 239 Declination circle, adjustment by, 237 facilitates setting up instrument, 110 Definition condition for excellence of, 254 good in situations widely different, 254 DeRheita, 12 constructed binoculars, 13 terrestrial ocular, 13 Descartes’ dioptrics, publication of, 11 lens with elliptical curvature, 12 Dew cap, 219 Diaphragms, importance of, 43