CHAPTER VII.--_Field glasses and telescopes._ Reflection 98 Refraction 98 Lenses 98 Focus 99 Optical center 99 Image 99 Conjugate foci 99 Law of foci 100 Formation of image 101 Spherical aberration 102 Chromatic aberration 102 Telescopes 104 Galilean field glasses and telescopes 106 Porro prism field glasses and telescopes 106 Field glasses 108 Properties of telescopes and field glasses 109 Power 109 Light 111 Field 114 Definition 115 Field glasses and telescopes issued by the Signal Corps 119 Type A 121 Type B 124 Type C 125 Type D 125 Field-glass specifications 126 CHAPTER I. INTRODUCTION.