Reproduced from _The Forces of Nature_ (Messrs. Macmillan)

THE GREAT SUN-SPOT OF JULY 17, 1905 22
Yerkes Observatory.

SOLAR PROMINENCES 22
From photographs taken at the Yerkes Observatory.

MARS, OCTOBER 5, 1909 23
Photo: Mount Wilson Observatory.

JUPITER 23

SATURN, NOVEMBER 19, 1911 23
Photo: Professor E. E. Barnard, Yerkes Observatory.

THE SPECTROSCOPE, AN INSTRUMENT FOR ANALYSING LIGHT; IT
PROVIDES MEANS FOR IDENTIFYING SUBSTANCES (_Coloured
Illustration_) 24

THE MOON 28

MARS 29
Drawings by Professor Percival Lowell.

THE MOON, AT NINE AND THREE QUARTER DAYS 29

A MAP OF THE CHIEF PLAINS AND CRATERS OF THE MOON 32

A DIAGRAM OF A STREAM OF METEORS SHOWING THE EARTH PASSING
THROUGH THEM 32

COMET, SEPTEMBER 29, 1908 33
Photo: Royal Observatory, Greenwich.

COMET, OCTOBER 3, 1908 33
Photo: Royal Observatory, Greenwich.

TYPICAL SPECTRA 36
Photo: Harvard College Observatory.

A NEBULAR REGION SOUTH OF ZETA ORIONIS 37
Photo: Mount Wilson Observatory.

STAR CLUSTER IN HERCULES 37
Photo: Astrophysical Observatory, Victoria, British Columbia.

THE GREAT NEBULA IN ORION 40
Photo: Yerkes Observatory.

GIANT SPIRAL NEBULA, MARCH 23, 1914 41
Photo: Lick Observatory.

A SPIRAL NEBULA SEEN EDGE-ON 44
Photo: Mount Wilson Observatory.

100-INCH TELESCOPE, MOUNT WILSON 45
Photo: H. J. Shepstone.

THE YERKES 40-INCH REFRACTOR 48

THE DOUBLE-SLIDE PLATE-HOLDER ON YERKES 40-INCH REFRACTING
TELESCOPE 49
Photo: H. J. Shepstone.

MODERN DIRE