Crystals of Iceland spar, double refraction by (_see_ Iceland Spar) Culmination of stars, first observations of, 5 Cycloidal pendulum, 185 D. Dawes, solar eyepiece, 114, 115, 349; photometry, 378 Day, solar and sidereal, 253, 254, 256 Day eyepiece, 113 Days, first reckoning of, 19; measurement of, 176 Dead-beat escapement, 198 Deal time-ball, 275, 279 Declination, 24, 234, 241, 243, 251; measured by Tycho Brahe, 45 Declination axis of the equatorial, 299, 308, 327, 328 Defining power of the modern telescope, 160, 164; stars in Orion a test of, 165 Degrees, division of the circle into, 8, 17, 21 De La Rue (Warren, F.R.S.), his reflecting telescope, 108; improvements in polishing specula, 134; celestial photography, 454, 459, 460, 464, 465, 475 Denderah, the zodiac of, 7 Dent (E. & Co.), clock at Royal Observatory, Greenwich, 194, 203, 204, 271, 274 Detached lever escapement, 208 Deviation of light, 79, 82 Deviation error in the transit instrument, 240, 248 Dials of ancient clocks, 257 Diagonal scale, Digges’, 213 Differential observations made with the equatorial, 367 Digges’ diagonal scale, 213 Diogenes Laertes, on the invention of the gnomon, 16 Dioptrics, Kepler’s treatise on, 386 Direct vision spectroscope, 431 Dispersion of light by prism, 79, 80, 82 Dividing power of telescopes, 165 Dollond, experiments with lenses, 85; correction of chromatic aberration, 89; on manufacture of flint-glass discs, 118; pancratic eyepiece, 113 Dome form of observatory, 338, 339 Double stars, 351, 359; measurement of, 360 Double-image micrometer, 225, 229 Double refraction by crystals of Iceland spar (_see_ Iceland Spar) Driving clock, for large telescopes, 318, 346 Drum form of observatory, 338 Dundee time signal, 278 E. Earth, The, its position in Ptolemy’s system, 3; early theories of its form, 6; circumference measured by Posidonius, 8; Euclid’s theory of its position, 12; inclination of its axis, 14, 17; size measured by Eratosthenes, 19; position in Tycho Brahe’s system, 46 Eclipses, first observations of, 4; eclipses of Jupiter’s moons; eclipses, solar, photograph of, 474 Ecliptic, plane of the, 13, 14; discovery of its inclination, 17; inclination measured by Eratosthenes, 19 Ecliptic astrolabe of Tycho Brahe, 28 Edinburgh Observatory, clock arrangements at, 269; standard clock, 272; time signals, 278 Egyptians, their record of eclipses, 4; zodiac of Denderah, 7 Eichens, his equatorial telescope at Paris, 314, 315; siderostat constructed by him, 344 Electricity, its application to the chronograph, 265; to driving and controlling clocks, 272 Electric lamp, 404; arranged for spectrum analysis, 405 Emery used in grinding lenses and specula, 127 English mounting of large telescopes, 310 Equation of time, 254 EQUATORIAL, THE (Book V.), 293-368 (_see_ Telescopes)