Prime-vertical instrument, 287 Primum mobile of Ptolemy, 3 Prisms, action of, 55; crown and flint-glass, 83, 84; water, 85; doubly refracting, for the micrometer, 226; direct vision, 400; in the spectroscope, 393-400; object-glass prism, 426 Ptolemy, the Heavens according to, 3; trigonometrical tables, 17; sun’s altitude, 21; his discoveries, 35; parallax of the moon, 35; his measurement of time, 36; parallactic rules, 38, 51 Purbach, observation of altitudes by, 36 Pyramids, the first constructed astronomical instruments, 5, 6 Q. Quadrants used by Tycho Brahe, 38; his _quadrans maximus_, 48 Quadrant, mural, 233, 235 Quartz crystals for polarizing light, 450, 452 R. Radiation of stars, visual, 383; thermal, 385 Radiation, general and selective, 403, 408 Ramsden’s eyepiece, 112, 212 Reading microscopes, for Greenwich and Cambridge (U.S.) transit circles, 247; for Newall’s telescope, 307 Red stars (_see_ Colour of Stars) Reflection of light (_see_ Light) Reflecting telescopes (_see_ Telescope) Reflective powers, Sir John Herschel’s table of, 168 Reflector, diagonal, for solar observations, 114 Reflecting and refracting telescopes compared, 170 Reflex zenith-tube at Greenwich, 286 Refracting telescopes (_see_ Telescopes) Refracting and reflecting telescopes compared, 170 Refraction of light (_see_ Light) Refraction, double, by crystals of Iceland spar (_see_ Iceland Spar) Refrangibility of colours, 387; of light, 420 Regiomontanus, altitudes measured by, 36 Regulation of clocks by electricity, 272 Rising of stars (_see_ Stars) Right ascension, 24, 234, 241, 249, 257; measured by Hipparchus, 44; by Tycho Brahe, 45 Ring micrometer, 368 Robinson (Dr.), specula of Melbourne telescope, 129; apertures of object-glasses, 168 Rockets fired as time signals, 281 Römer, wires in a transit eyepiece, 220; transit circle and transit instrument, 284 Rosse (Lord), his reflecting telescope, 108, 294, 311, 312; composition of reflector, 120; production of metallic specula, 121, 131; nebula of Orion as seen by his reflector, 157, 158; illuminating power of his telescope, 159; effective light, 169; thermopile observations, 384 Royal Observatory, Greenwich (_see_ Greenwich) Rudolph II. (Emperor), his patronage of Tycho Brahe, 42 Rumford’s photometer, 377 Rutherfurd, his work in celestial photography, 455, 464, 466, 471, 477, 480 S. Salts, flame of, in a Bunsen’s burner, 407 Sand clocks and sand glasses, 176 Saturn, in Ptolemy’s system, 3; in Tycho Brahe’s, 46; as seen with a 3¾ inch and 26 inch object-glass, 160, 161; as a telescopic object, 351; mode of measuring its rings, 357; photographs of, 465, 466 Savart’s analyser for polarization of light, 452 Scarphie, employed by Eratosthenes, 19 Scheiner’s telescope, 78 Seasons, The, 15, 16