changes. But successful photographs of these objects have been possible only during the past twenty years; while extremely sharp photographs, suitable for the detection of small changes, have been secured only during the past five or six years. Furthermore, the reflecting telescopes which have been used in this work thus far are of comparatively small focal length, so that the scale of the photographs is small. Hence it is not strange that no changes of form have yet been detected with certainty in any nebula.
When a really great reflecting telescope—one comparable in size and cost with the largest modern refractors—is constructed for this work, and is employed systematically, in a fine climate and for a long term of years, in securing the extremely sharp photographs of large scale, which would now unquestionably be possible with such an instrument, we shall certainly be able to secure some definite information concerning the real significance of these mysterious objects, and upon the whole stupendous problem of the development of the nebulæ and stars.
There are many other fine examples of clusters of stars involved in nebulosity. I shall mention only one other—a beautiful group known as Messier 8, in the constellation of Sagittarius. The cluster and nebula are together sufficiently bright to be seen by the unaided eye, as a hazy patch of light in the southern Milky Way. This cluster is about 24° south of the celestial equator, and hence is so low that it is photographed with difficulty from our northern latitude. In the summer of 1903, on an extraordinarily brilliant and quiet night, the writer secured a photograph of this object which shows an immense number of stars involved in a large nebula as rich and intricate in structure as that in the Pleiades. Perhaps the most remarkable peculiarity of this nebula is the presence of several "dark holes," so black, and with edges so clear-cut, as to suggest the presence of obstructing masses between us and the nebula. Many examples of such dark holes and rifts have been found, particularly in this immediate region of the Milky Way and in the neighboring constellations of Scorpio and Ophiuchus.
It is a suggestive fact that the more open or coarse clusters of stars are, in general, the ones which are involved in nebulosity. Can it be that the Pleiades, Messier 8, and similar clusters are examples of an early stage of evolution, in which the stars have been developed out of the nebulosity comparatively recently, are still developing, and have not yet become massed closely together through the influence of gravitation?
We now come to the class which I shall call the moderately dense clusters. Many fine examples of these have been photographed with the two-foot reflector and with the 40-inch refractor. Notable among them are several magnificent clusters which are visible to the unaided eye as hazy patches of light. One of them, the well-known "beehive" cluster in the constellation of Cancer, is the Præsepe of the ancients. Galileo was able to see thirty-six stars in this cluster with his first telescope; modern telescopes and photographs show many hundreds. Another example of this class is the great double cluster in the constellation of Perseus, one of the most splendid telescopic objects in the heavens. With a low magnifying power both of these superb masses of stars can be seen in the same field of view. In the photographs a comparatively large region of the sky is included on one plate, and the contrast between these dense, brilliant masses and the comparatively thin region about them is most striking.
The telescope and the photographs reveal a great number of clusters of this class which are entirely invisible to the unaided eye. In the constellation of Antinous, in the Milky Way, is a most beautiful telescopic cluster known as Messier 11. Although not very brilliant, it affords a fine illustration of the massing of stars into clusters; for this massing is most conspicuous, even though this cluster is in one of the very dense regions of the Milky Way. Another similar cluster, but larger and richer, is the superb one known as Messier 37, in the constellation of Auriga. Messier 52 in the constellation of Cepheus, and Messier 71 in the constellation of Sagitta, are other fine examples of clusters of this class.
We now come to the globular star-clusters. With the exception of the nebulæ these may well be regarded as
