Photographs of Nebulæ and Clusters, Made With the Crossley Reflector

Part 1

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NOTE.

In the original negatives of subjects 10 and 12, there are faint dark rings immediately surrounding some of the stars in the denser parts of the nebulosity. This effect has no doubt been accentuated in the subsequent photographic processes. On the plates of these two subjects in the completed volume, these rings are very distinct and give rise to a suspicion that the effect has been enhanced by the engraver. A critical examination of the prints seems to confirm this view. In the original proofs these rings were inconspicuous and were not noticed. The processes of steel-facing and printing appear to have increased the effect markedly, as it is much stronger on the sheets printed for the edition than in any of the early proofs.

Inasmuch as these effects were not and could not be discovered until the sheets were assembled in Sacramento for binding, it has not been thought desirable to delay the issue of the volume for several weeks additional in order to have new plates and new prints of these subjects made by the distant engraver.

Lick Observatory, Mount Hamilton, November, 1908.

THE GREAT NEBULA IN ORION]

UNIVERSITY OF CALIFORNIA PUBLICATIONS

PUBLICATIONS OF THE LICK OBSERVATORY

PRINTED BY AUTHORITY OF THE REGENTS OF THE UNIVERSITY

VOLUME VIII

SACRAMENTO W. W. SHANNON SUPERINTENDENT OF STATE PRINTING 1908

AS A TRIBUTE TO THE MEMORY OF

JAMES EDWARD KEELER

and in recognition of his great worth as a man and as an astronomer, the plates for this volume have been provided by

MR. WILLIAM ALVORD, MR. F. M. SMITH, MR. ROBERT BRUCE, MISS JENNIE SMITH, MR. WILLIAM H. CROCKER, MISS MATILDA H. SMITH, MRS. WILLIAM H. CROCKER, MR. BENJAMIN THAW, MR. E. J. DE SABLA, MRS. WILLIAM THAW, MR. J. A. DONOHOE, MR. ROBERT J. TOBIN, MRS. PHOEBE A. HEARST, THE UNIVERSITY OF CALIFORNIA, MR. JOHN B. JACKSON, THE STATE OF CALIFORNIA. MR. E. J. MOLERA,

ORGANIZATION OF THE LICK OBSERVATORY.

HON. CHARLES W. SLACK, HON. WARREN R. PORTER,

HON. WILLIAM H. CROCKER, REV. PETER C. YORKE,

Committee of the Regents for the Lick Observatory.

BENJAMIN IDE WHEELER, President of the University.

W. W. CAMPBELL, Director and Astronomer.

R. H. TUCKER,* Astronomer.

C. D. PERRINE, Astronomer.

H. D. CURTIS, Mills Acting Astronomer.

R. G. AITKEN, Astronomer.

W. H. WRIGHT, Astronomer.

J. H. MOORE, Assistant Astronomer.

SEBASTIAN ALBRECHT, Assistant Astronomer.

Part 2

MISS A. M. HOBE, Carnegie Assistant.

G. F. PADDOCK, Mills Assistant.

MISS L. B. ALLEN, Carnegie Assistant.

E. A. FATH, Fellow.

J. C. DUNCAN, Fellow.

MISS A. E. GLANCY, Fellow.

MISS M. E. FRENCH,* Secretary.

MISS A. J. VAN COOVER, Secretary.

* Absent on leave.

PHOTOGRAPHS OF NEBULÆ AND CLUSTERS, MADE WITH THE CROSSLEY REFLECTOR,

BY JAMES EDWARD KEELER, DIRECTOR OF THE LICK OBSERVATORY.

1898-1900.

PREFACE.

When Professor Keeler entered upon the duties of Director of the Lick Observatory, on June 1, 1898, he planned to devote his observing time for several years to photographing the brighter nebulæ and star clusters, with the Crossley reflector. The story of his wonderful success with this difficult instrument is familiar to all readers of astronomical literature: this form of telescope was in effect born again; and his contributions to our knowledge of the nebulæ were epoch-making.

Professor Keeler's observing programme included one hundred and four subjects. At the time of his lamented death, on August 12, 1900, satisfactory negatives of two-thirds of the selected objects had been secured. The unphotographed objects were mainly those which come into observing position in the unfavorable winter and spring months. The completion of the programme was entrusted to Assistant Astronomer Perrine. The observers were assisted chiefly by Mr. H. K. Palmer, and in smaller degree by Messrs. Joel Stebbins, C. G. Dall, R. H. Curtiss and Sebastian Albrecht.

Professor Keeler's photographs enabled him to make two discoveries of prime importance, not to mention several that are scarcely secondary to them.

1st.--"Many thousands of unrecorded nebulæ exist in the sky. A conservative estimate places the number within reach of the Crossley reflector at about 120,000. The number of nebulæ in our catalogues is but a small fraction of this." [The number already discovered and catalogued did not exceed 13,000. Later observations with the Crossley reflector, with longer exposure-times and more sensitive plates, render it probable that the number of nebulæ discoverable with this powerful instrument is of the order of half a million.]

2d.--"Most of these nebulæ have a spiral structure."

The photographs of the one hundred and four subjects contain the images of 744 nebulæ not previously observed. A catalogue of these is published in the present volume. Their positions, which are thought to be accurate within 1´´, were determined by Messrs. Palmer, Curtiss, and Albrecht.

The main purpose of this volume is to reproduce and make available for study, the larger and more interesting nebulæ and clusters on the programme, sixty-eight in number. The thirty-six subjects not reproduced are for the most part small or apparently not of special interest. The difficulties attending the reproduction of astronomical photographs by mechanical processes are well-known to all who have made the attempt. It seems necessary to recognize, at least at present, that delicate details of structure will be lost, and that contrasts between very bright and very faint regions will be changed, especially if a good sky background is preserved; in other words, that the best obtainable reproductions fall far short of doing justice to the original photographs. Technical studies should be based upon the original negatives or upon copies on glass.

After considerable experimental work, involving several methods and several firms, the making of the heliogravure plates and the hand-press prints was entrusted to The Photogravure and Color Company of New York City. To this firm's continued interest and willingness to act on constructive criticism is due much of the excellence of the results.

The expensive reproductions could hardly have been undertaken without the generous assistance of the donors mentioned on a preceding page.

Professor Keeler's description of the Crossley reflector, of his methods of observing, and of the chief results obtained, was written only a short time before his death. It is here republished. Other results of his work are described in the several papers to which the footnotes refer.

TABLE OF CONTENTS.

The Orion Nebula, Frontispiece

The Crossley Reflector of the Lick Observatory, Page 11

List of Nebulæ and Clusters Photographed, " 30

Catalogue of New Nebulæ Discovered on the Negatives, " 31

Part 3

Positions of Known Nebulæ Determined from the Crossley Negatives, " 42

List of Illustrations, " 45

Illustrations, following " 46

THE CROSSLEY REFLECTOR OF THE LICK OBSERVATORY.

By JAMES E. KEELER.

The Crossley reflector, at present the largest instrument of its class in America, was made in 1879 by Dr. A. A. Common, of London, in order to carry out, and test by practical observation, certain ideas of his respecting the design of large reflecting telescopes. For the construction of the instrument embodying these ideas, and for some fine astronomical photographs obtained with it, Dr. Common was awarded the gold medal of the Royal Astronomical Society in 1884.

In 1885, Dr. Common, wishing to make a larger telescope on a somewhat similar plan, sold the instrument to Edward Crossley, Esq., F. R. A. S., of Halifax, England. Mr. Crossley provided the telescope with a dome of the usual form, in place of the sliding roof used by its former owner, and made observations with it for some years; but the climate of Halifax not being suitable for the best use of such a telescope, he consented, at the request of Dr. Holden, then Director of the Lick Observatory, to present it to this institution. The funds for transporting the telescope and dome to California, and setting them up on Mount Hamilton, were subscribed by friends of the Lick Observatory, for the most part citizens of California. The work was completed, and the telescope housed in a suitable observatory building, in 1895.

On taking charge of the Lick Observatory in 1898, I decided to devote my own observing time to the Crossley reflector, although the whole of my previous experience had been with refracting telescopes. I was more particularly desirous of testing the reflector with my own hands, because such preliminary trials of it as had been made had given rise to somewhat conflicting opinions as to its merits. The result of my experience is given in the following article, which is written chiefly with reference to American readers. If I have taken occasion to point out what I regard as defects in the design or construction of the instrument, I have done so, not from any desire to look a gift horse in the mouth, but in the interest of future improvement, and to make intelligible the circumstances under which the work of the reflector is now being done and will be done hereafter. The most important improvements which have suggested themselves have indeed already been made by Dr. Common himself, in constructing his five-foot telescope. The three-foot reflector is, in spite of numerous idiosyncracies which make its management very different from the comparatively simple manipulation of a refractor, by far the most effective instrument in the Observatory for certain classes of astronomical work. Certainly no one has more reason than I to appreciate the great value of Mr. Crossley's generous gift.

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