Section Iv.—the Sidereal Heavens.

Frontispiece—NEBULA IN ANDROMEDA 31 MESSIER 398

1.— STARS VISIBLE IN THE NORTHERN HEMISPHERE 401 2.— STARS VISIBLE IN THE SOUTHERN HEMISPHERE 403 3.— DIAGRAM SHOWING “SOLAR APEX” AS ROUNDLY COMPUTED 429 4.— APPARENT ORBIT OF ZETA HERCULIS 436 5.— APPARENT ORBIT OF THE COMPANION OF SIRIUS 439 6.— APPARENT ORBIT OF 70 OPHIUCHI 443 7.— TRIPLE STARS 451 8.— THE TEMPORARY STAR OF 1572 481 9.— THE DOUBLE STAR CLUSTER IN PERSEUS 503 10.— STAR CLUSTER IN GEMINI 504 11.— 37 MESSIER 505 12.— STAR CLUSTER IN HERCULES 507 13.— THE STAR CLUSTER, OMEGA CENTAURI 512 14.— THE ORION NEBULÆ 521 15.— THE NEBULA ROUND ETA ARGUS 523 16.— THE TRIFID NEBULA, SAGITTARIUS 525 17.— SPIRAL NEBULA, 51 MESSIER 533 18.— MAGELLANIC CLOUDS 537 19.— PHOTOGRAPH OF MILKY WAY, SAGITTARIUS 555 20.— THE MILKY WAY 557

ASTRONOMY

THE OBSERVATORY AT NICE.