vatory, 258

Prism, 121

Prismatic colours, 111, 121

Procyon, 284, 290, 297, 303

Prominences, Solar, 72, 93, 131, 139-140, 143;
first observation of, with spectroscope, 94, 140, 236

Proper motions of stars, 126, 281-285, 326, 329-330

Ptolemaeus (lunar crater), 198-199, 204

Ptolemaic idea, 319;
system, 18, 19, 158, 171-172

Ptolemy, 18, 101, 171, 290, 296

Puiseux, P., 206

Pulkowa telescope, 117

Puppis, V., 310

Quiescent prominences, 139

Radcliffe Observer, 139

"Radiant," or radiant point, 269

Radiation from sun, 130, 134

Radium, 129, 138

Rainbow, 121

"Rainbows, Bay of," 197

Rambaut, A., 139

Ramsay, Sir William, 138

Rays (on moon), 204

Recurrence of eclipses, 74-80

Red (rays of light), 121, 125, 127, 130

Red Spot, the Great, 230

Reflecting telescope, 111-116;
future of, 119

Reflector. _See_ Reflecting telescope

Refracting and reflecting telescopes contrasted, 118

Refracting telescope, 109-111, 115-117;
limits to size of, 119-120

Refraction, 121, 168-169

Refractor, _See_ Refracting telescope

Regulus, 290, 296

Retrograde motion of Phoebe, 240, 250, 336

"Reversing Layer," 94, 130, 132, 137-138

Revival of learning, 107

Revolution, 30;
of earth around sun, 170-173;
periods of sun and planets, 35

Riccioli, 198

Rice-grain structure of photosphere, 134

Rigel, 285, 297

Rills (on moon), 204

Ring-mountains of moon. _See_ Craters

"Ring" nebulae, 315, 337

"Ring with wings," 87

Rings of Saturn, 108, 236-239, 241-243, 334

Ritchey, G.W., 118

Roberts, A.W., 308, 310

Roberts, Isaac, 325

"Roche's limit," 238

Roemer, 235

Roman history, eclipses in, 85-86

Romulus, 85

Roentgen, 120

Rosse, great telescope of Lord, 117, 314

Rotation, 30;
of earth, 33, 161-165, 170;
of sun, 34, 125, 135-136, 231;
periods of sun and planets, 35

Royal Society of London, 90-91, 111

Rubicon, Passage of the, 85

"Runaway" stars, 281, 326, 330

Sagittarius (constellation), 316

Salt, spectrum of table, 122

Samarcand, 107

"Saros," Chaldean, 76-78, 84

Satellites, 26-27, 37

Saturn, 20, 22, 34, 37, 108, 236-243, 258;
comet family of, 252;
a puzzle to the early telescope observers, 241-243;
retrograde motion of satellite Phoebe, 240, 250, 336;