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270 our means of observation and as an example of how the standard of such work has been gradually raised. It will be interesting to trace in the same way the history of the Horseshoe Nebula in Sagittarius, which, next to the great nebulosities of Orion and Andromeda, is the most curious of these objects, and which perhaps as much as any other deserves careful study.

Its discovery dates back about a hundred years to the time of Messier, the assiduous astronomer of the Observatoire de la Marine at Paris; it is No. 17 of his list, which comprises most of the brighter and more remarkable nebulæ of the northern sky. It was at this time that Sir William Herschel, the famous astronomer of England, with instruments far superior in power to those of Messier, was forming his great catalogues of the nebulæ discovered in his "sweeps." Messier wisely used his smaller instrument in the endeavor to obtain accurate positions for those found by him, and he has left us monographic studies of the Orion and the Andromeda nebula ("Mémoires de l'Académie des Sciences," 1771 and 1807), which are almost the first trustworthy works of the kind, and which are the beginnings from which sprang the elaborate drawings of Lassell, Rosse, Struve and Bond.



From the time of Messier to 1826, when Sir John Herschel published his first figure of the Orion nebula, almost nothing was done in this line of research; but in 1833 a study of the Horseshoe Nebula was published by Sir John Herschel, together with many other similar drawings, in the "Philosophical Transactions" (see Fig. 1). This was the first considerable and systematic attempt to accurately figure the nebulae, and it doubtless turned the attention of astronomers generally to this branch, the importance of which was manifest. If so many of the fixed stars changed in brilliancy and in position, why should not