ILLUSTRATIONS

SIMON NEWCOMB

PHOTOGRAPH OF THE CORONA OF THE SUN, TAKEN IN TRIPOLI DURING TOTAL ECLIPSE OF AUGUST 30, 1905.

A TYPICAL STAR CLUSTER-CENTAURI

THE GLASS DISK

THE OPTICIAN'S TOOL

THE OPTICIAN'S TOOL

GRINDING A LARGE LENS

IMAGE OF CANDLE-FLAME IN OBJECT-GLASS

TESTING ADJUSTMENT OF OBJECT-GLASS

A VERY PRIMITIVE MOUNTING FOR A TELESCOPE

THE HUYGHENIAN EYE-PIECE

SECTION OF THE PRIMITIVE MOUNTING

SPECTRAL IMAGES OF STARS, THE UPPER LINE SHOWING HOW THEY APPEAR WITH THE EYE-PIECE PUSHED IN, THE LOWER WITH THE EYE-PIECE DRAWN OUT

THE GREAT REFRACTOR OF THE NATIONAL OBSERVATORY AT WASHINGTON

THE "BROKEN-BACKED COMET-SEEKER"

NEBULA IN ORION

DIP OF THE MAGNETIC NEEDLE IN VARIOUS LATITUDES

STAR SPECTRA

PROFESSOR LANGLEY'S AIR-SHIP

PREFACE

In preparing and issuing this collection of essays and addresses, the author has yielded to what he could not but regard as the too flattering judgment of the publishers. Having done this, it became incumbent to do what he could to justify their good opinion by revising the material and bringing it up to date. Interest rather than unity of thought has determined the selection.

A prominent theme in the collection is that of the structure, extent, and duration of the universe. Here some repetition of ideas was found unavoidable, in a case where what is substantially a single theme has been treated in the various forms which it assumed in the light of constantly growing knowledge. If the critical reader finds this a defect, the author can plead in extenuation only the difficulty of avoiding it under the circumstances. Although mainly astronomical, a number of discussions relating to general scientific subjects have been included.

Acknowledgment is due to the proprietors of the various periodicals from the pages of which most of the essays have been taken. Besides Harper's Magazine and the North American Review, these include McClure's Magazine, from which were taken the articles "The Unsolved Problems of Astronomy" and "How the Planets are Weighed." "The Structure of the Universe" appeared in the International Monthly, now the International Quarterly; "The Outlook for the Flying-Machine" is mainly from The New York Independent, but in part from McClure's Magazine; "The World's Debt to Astronomy" is from The Chautauquan; and "An Astronomical Friendship" from the Atlantic Monthly.

SIMON NEWCOMB. WASHINGTON, JUNE, 1906.