Ring micrometer, 169 computation of results of, 170 Ring system faults due to strain, 205 “Romsey” observatory type, 248 Rack motion in altitude, 100 Ramsden, ocular, 144 Reflection, coefficient of, from silvered surface, 54 Reflector costs, 55 cover for, 242 development in England, 41 for astrophysical work, 56 light-grasp of, 53 relative aperture of, 50 section of Newtonian, 45 skeleton construction, 49 suffers from scattered light, 56 working field of, 55 Refractive index, 63 Refractors and reflectors, relative advantages of, 52 few made after advent of reflector, 27 in section, 43 light transmission of, 53 Refractors, relative equivalent apertures of, 54 tubes of, 42 S Scheiner, Christopher, use of Kepler’s telescope, 11 devised parallactic mount, 11 Secondary spectrum, 87 new glasses reducing, 88 Seeing, 257 conditions, for difference of aperture, 257 conditions generally bad, 253 standard scale of, 256 true inwardness of bad, 253 Separating power, to compute, 261 Short, James, mastered art of figuring paraboloid, 27 took up Gregorian construction with success, 27 Shortened telescope, 152 Sights, on portable mount, 229 Silver films, condition of, 54 Silvering, Ludin’s process, 225 processes, 222 process, Dr. Brashear’s, 222 Sine condition, Abbé’s, 82 Slit, spectroscope, Abbé type, 184 Snow cœlostat telescope, 127 Solar diagonal, 166 eye piece diaphragms in, 168 early spectroscopes, 188 polarizing eyepiece, 167 spectroscope, 187 Spacers, 44, 218 Spectacle lenses, combination of, 2 Spectacles for presbyopia, 2 invention of, 1 Spectra, visibility of stellar, 183 Spectro-heliograph, principle of, 191 simple type of Hale’s, 191 Spectroscope, 182 construction of astronomical, 182 of Lowell refractor, 185 ocular, McClean form, 183 Specula, small, methods of support, 49 Speculum metal composition of, 24 Sphenoid prisms, 158, 163 Spherical aberration, 11 amount of, 80 annulling in both directions, 84 examination for, 207 quick test of, 267 remedy for, 79 concave mirror, errors of, 22 Star, appearance of, 204 artificial, 66, 203 diagonal, 165 disc, apparent diameter of, 259 image of reflector, 206 Steinheil, achromatic ocular, 144 Karl August, silvering specula, 39 Striæ, location of, 67 Surface, treatment of deterioration of, 218 T Taylor, triplets with reduced secondary spectrum, 89 Telescopes, choice and purchase of, 201 Early in 1610 made in England, 6 first, 3 the first astronomical, 9 improvement of early, 11 lineage of, 1 name devised, 9 Telescopes, portable and fixed, 108 1609, for sale in Paris, 5 size and mounting of early, 14 Telescopic vision, discovery of, 2 Templets, designed curves of, 69