Part 21

Railroad time, explained, 127

Refraction, atmospheric, explained, 193

"Regulator," the jeweller's clock, 114

Ring-nebulæ, 31

Rings, of Saturn, see Saturn's rings.

Roberts, Andromeda nebula, 28

Rotation, of Earth, 136, 162, 171, 184 of Saturn, 150

Royal Astronomer, his duties, 2

Royal Observatory, Greenwich, 124 Greenwich, Bradley's observations, 219 Naples, latitude observations, 139

Rutherfurd, cluster photography, 99 invents photographic apparatus, 93 pioneer in photography, 90 stellar parallax, 94

Sagredus, character in Galileo's Dialogue, 55

Salusbury, Galileo's translator, 50, 54

Salviati, character in Galileo's Dialogue, 55

Samoa, its date, 126

SATURN'S RINGS, 140 analogy to planetoids, 147 announced by Huygens, 142 observed with spectroscope, 147 shown to be double by Cassini, 144 structure and stability, 145

Scales, of heliometer, 158

Scorpio, constellation, temporary star in, 39

Semi-lenses of heliometer, 155

Sextant, how used, 4

Sicily, latitude station in, 139

Sidereus Nuncius, published by Galileo, 52

Simplicio, character in Galileo's Dialogue, 55

Sirius, brightest star, 205

Size of Moon, measured, 166

Société de l'Optique, 177

Solar parallax, see Sun's distance. physics, by photography, 109 system, stability of, 210

Spectroscope, its use explained, 147 used on pole-star, 19 to observe Saturn's rings, 147

Spiral nebulæ, 31