In considering the appearance of the dark objects in the photographs, the reader should bear in mind the nature of the instruments with which they were taken. Figs. 1, 6, 8, and 9 show very large regions of the sky on a small scale, taken with a camera of 50 inches focal length, provided with a large portrait lens of 10 inches aperture (the Bruce telescope). Figs. 5, 7, 10, and 12, on the contrary,

some purposes, the latter has the advantage of bringing before us a large region of the sky in a single photograph, and of accentuating the blackness and definiteness of objects that might not be detected with a large telescope because of their diffuse character in long-focus instruments. When once recognized on the portrait lens plates, however, their detailed study with a large telescope becomes essential. Even

under the searching scrutiny of the 100inch reflector, the sharpness and blackness of some of the dark nebulæ are so great that their images remain very distinct, as in the case of Barnard's No. 86 (Fig. 5). But in other instances such strong contrast is lacking, or the object may be so large as to cover the entire field of the 100-inch telescope.

In the region about the bright star Theta Ophiuchi many dark markings, both large and small, are found (Fig. 6). Some of these are very peculiar in form, and do not resemble any of the bright nebulæ. Nevertheless the evidence indicates that most of them are dark, obscuring masses, which cut off the light of the distant stars and leave visible only those that lie between the dark masses and the earth. The S-shaped area in the middle of Fig. 6 is Barnard's No. 72, shown on a

large scale in Duncan's photograph with the 100-inch telescope (Fig. 7). Another fine region of the Milky Way photographed by Barnard is that in Cepheus, where many dark objects also appear (Fig. 8). Some of these are superposed on the great bright nebula that occupies the central area of the picture.

Thus far we have been considering the darker areas, some of which appear inky black when observed visually with a telescope. It is evident that if these are actually obscuring masses, lying between us and the more distant stars, they should be most conspicuous where the luminous background is brightest and most continuous, as in the crowded regions of the Milky Way. Barnard's photographs show this to be the case. With a very large telescope many stars too faint to be recorded on the Bruce photographs are seen,

ous in the great clouds of the Milky Way, but they are comparatively few in number in the sky away from this region. Nevertheless, Barnard has found many dark objects in such areas of few stars. He gives as a notable instance No. 15 in his catalogue, elliptical in form, its longer axis equal to half the moon's diameter. "The background on which the stars shine is uniform over the entire plate. The object is in a region somewhat larger than itself, where there are relatively few stars, and is black by contrast with the sky alone.

FAINTLY LUMINOUS NEBULE

Without attempting in the present article to discuss this feebly luminous background of the entire heavens, we may turn to the examination of obscuring nebulæ which, though they blot out the stars beyond them, seem to be slightly luminous themselves. Barnard's No. 92, the apparently black elliptical object in the centre of Fig. 9, is of this class. Lying against a bright star cloud in Sagittarius, it resembles on the Bruce photograph the

completely surrounds the entire constellation, while the bright nebula in the sword-handle, though small by comparison, is one of the most striking of telescopic objects. Thanks to the large reflectors, we find that the most remarkable of the dark nebula is also in Orion. Discovered long ago, the peculiar object shown in Duncan's photograph (Fig. 12) is too small to be effectively photographed with a short-focus telescope. It is evidently part of an immense obscuring mass, which blots out most of the stars

on the left of the picture and projects, with its luminous border, in front of the star-filled area on the right. This photograph also shows a bright nebula on the left, above the centre of the cut.

Space is lacking to describe the hundreds of careful tests, both visual and photographic, which finally convinced Barnard of the existence of dark nebulæ. The value of his testimony is enhanced by his scepticism in 1894, when Mr. A. C. Ranyard reproduced one of Barnard's Lick Observatory photographs in his