Diffraction figure for bright line, 269 pattern, 256 solid, apparent diameter of, 262 solid of planet, 269 solid for a star, 260 spectra, 190 system, scale of, 260 varies inversely with aperture, 260 through objective, 258 Digges, account suggests camera obscura, 7 Dimensions, customary, telescope of, 24 Discs, inspection of glass, 66 roughing to form, 69 Distortion, 86 Dolland, John, 28 published his discovery of achromatism, 29 Peter, early triple objective, 29 Dome wholly of galvanized iron, 250 Domes, 246 Driving clock, a simple, 174 pendulum controlled, 177 clocks spring operated, 175 E English equatorial, 110 mounts, mechanical stability of, 113 Equatorial, adjustments of, 230 Equatorial, coudé, 124 mount, different situations in using, 229 mount, first by Short, 104 mount, pier overhung, 115 mount in section, 107 two motions necessary in, 106 Equilibrating levers, devised by T. Grubb, 39 Evershed, direct vision solar spectroscope, 189 Eye lens, simple, preferred by Sir W. Herschel, 136 Eyepiece, compensating, 142 Huygenian, 139 Huygenian, achromatism of, 140 Huygenian, with cross wires, 140 Huygenian, field of, 141 Huygenian focal length of, 143 measuring focus of, 136 microscope form, 147, 148 monocentric, 139 a simple microscope, 134 Tolles solid, 141 F Field, curvature of, 85 glass, arrangement of parts, 151 Galilean, 150 lens diameter possible, 150 Field lens, 139 Figuring locally, 73 process of, 73 Filar micrometer, 172 Finder, 108, 132 adjustment of, 230 Fine grinding, 69 Fixed eyepiece mounts, 118 Flints, highly refractive due to Guinand, 36 Foucault, 39 development of silver on glass reflector, 41 knife edge test, 212 Foucault, methods of working and testing, 41 Fraunhofer, 36 applied condition of absence of coma, 82 form of objectives, 37 long list of notable achievements, 38 “Front view” telescope, 32 mechanical difficulty of, 33 Furnaces, glass, classes of, 59 G Galilean telescope, small field of, 9 Galileo, exhibited telescope to senators of Venice, 8 grasps the general principles, 7 produces instrument magnifying 32 times, 8 Gascoigne, William, first using genuine micrometer, 12 Gauss, Objective, 82 Gerrish, application of drive, 181 motor drive, 179 Ghosts, 137 Glass, dark, as sunshade, 166 forming and annealing, 62 inspection of raw, 61 losses by volatilization, 58 materials of, 59 origin of, 57 persistent bubbles in, 58 a solid solution, 57 Grating spectroscopes, 190 Gratings, spectroscope, 189 Gregory, James, described construction which bears his name, 19 failed of material success, 20 Grubb, Sir Howard, objectives, 74 Guinand, Pierre Louis, improvements in optical glass, 36 H