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PART II

A STUDY OF OCCULTING MATTER IN THE

SPIRAL NEBULAE

By HEBER DOUST CURTIS

ASTRONOMER IN THE LICK OBSERVATORY

UNIVERSITY OF CALIFORNIA PUBLICATIONS
PUBLICATIONS OF THE LICK OBSERVATORY, VOL. XIII, PART II

A STUDY OF OCCULTING MATTER IN THE
SPIRAL NEBULAE

By HEBER DOUST CURTIS

It has long been a matter of common knowledge that certain spiral nebulae seen edgewise show a dark lane running down the length of the spiral, an appearance generally explained as due to a band of absorbing or occulting matter. The Crossley photographs at present number nearly five hundred spirals; this total includes only those spirals of sufficient size to show some structural detail and omits a large number of very small or very faint nebulae. It is the purpose of this paper to show that the occurrence of such dark bands in the spiral nebulae is a relatively common phenomenon. This fact can scarcely fail to be of great importance in the study of this highly interesting and still but imperfectly understood class of celestial objects. Inasmuch as any description of these complex bodies is difficult and unsatisfactory, the evidence as to the dark lanes must be almost entirely pictorial; it here consists of reproductions of seventy-seven spiral nebulae. It should be emphasized that the evidence can not be considered as in any way complete, for new examples illustrating the phenomenon are continually being recorded, and there are still several hundred large or bright spirals not yet photographed on the Crossley nebular programme.

The attempt has been made to group together in the illustrations nebulae of the same general type, but there are some exceptions to this order, due to the addition of objects after several of the half-tone blocks had been made. To secure compactness, the true orientation of the nebulae has been disregarded and all have been placed with the major axis horizontal, and in general, with any dark lanes underneath. Different degrees of enlargement were used, depending upon the character and size of the images on the original negatives. The following table contains the data as to the enlargement, the exposure time, and the true orientation of each nebula, and will also serve to identify the illustrations.

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