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THE POPULAR SCIENCE MONTHLY.

and devoting all his energies to his chosen science in a city which was then the most famous in the world for its astronomical instruments; and the other, a mere hoy, oppressed by narrow circumstances, working in the intervals of his college duties with a telescope which he had himself constructed, with a fellow-student (Mr. Hamilton L. Smith) as his only assistant.

The work of both astronomers (for it is impossible to deny to Mason that title) is of great excellence, but it will not be claiming too much to assert that Mason's was by far the most valuable monographic study of a nebula which had appeared, and indeed, in its thorough appreciation of the problems to be solved and in its most skillful adaptation of the existing means toward that end, it deserves to rank with the greatest works of this class, with Bond's, Lassell's, Rosse's and Struve's. It is not only in the observations themselves nor in the exquisite and accurate drawings which accompany the memoir that we feel this excellence, but in the philosophical grasp of the whole subject and the masterly appreciation of the fundamental ideas of the problem. His memoir contains so much that bears on this general aspect, that we quote from it largely, as it is too little known among those not professional astronomers:

"Although a period of nearly fifty years has now elapsed since the researches of the elder Herschel exposed to us the wide distribution of nebulous matter through the universe, we are still almost as ignorant as ever of its nature and intention. The same lapse of time that, among his extensive lists of double stars, has revealed to us the revolution of sun around sun, and given us a partial insight into the internal economy of those remote sidereal systems, has been apparently insufficient to discover any changes of a definite character in the nebulæ, and thereby to inform us at all of their past history, the form of their original creation, or their future destiny. At the same time, the detection of such changes is in the highest degree desirable, since no other sources of evidence can be safely relied upon in these inquiries. That the efforts of astronomers have thus far ended, at least, in vague and contradictory conjectures, is principally attributable to the great difficulty of originally observing, and of describing to future observers, bodies so shapeless and indeterminate in their forms, with the requisite precision. For we cannot doubt, authorized as we are to extend the laws of gravitation far into the recesses of space, that these masses of diffused matter are actually undergoing vast revolutions in form and constitution. The main object of this paper is to inquire how far that minute accuracy which has achieved such signal discoveries in the allied department of the double stars may be introduced into the observation of nebulæ, by modes of examination and description more peculiarly adapted to this end than such as can be employed in general reviews of the heavens. . . . It will conduce to a clearer understanding of our object to point out, generally and rapidly, the distinctions between our own theory of observation and that commonly adopted. It consists not in an extensive review, but in confining the attention to a few individuals; upon these exercising a long and minute scrutiny, during a succession of evenings; rendering even the slightest particulars of each nebula as precise as repeated observation and comparison with varied precautions can make them, and confirming each more doubtful and less legible of its features by a repetition of suspi-

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