86 UNIVEKSITY OF VIRGINIA PUBLICATIONS 



ascension were estimated to tenths of seconds of time and those in declina- 

 tion to "parts," the value of each "part" being approximately 0."2. Even 

 the brighter stars were liable to an error of at least one or two of the units 

 in which their positions were read, and a relatively large error must be 

 expected for the fainter stars. 



In the years 1847-8 Professor W. C. Bond, at Hai-\'ard College Observa- 

 tory, made a series of micrometrioal observations of 96 stars in the nebula. 

 [H. C. 0. Annals, Vol. V.] In most cases the relative positions of the stars 

 are based on but one or two measures, and consequently have large probable 

 errors. 



The work of Lassell, published in the Memoirs of the Eoyal Astronomi- 

 cal Society, 1854, contains positions of 59 of the brighter stars. The 

 position angles with reference to ® were read to minutes and the distances 

 to seconds of arc, so that the probable error of all the positions is quite 

 large. 



More accurate as well as more extensive were the later surveys of Liapun- 

 ofE and Struve, St. Petersburg, 1862, and of G. P. Bond, at Harvard 

 College Observatory, 1859-65 [H. C. 0. Annals, Vol. V] . The work of 

 Professor Bond was an extensive survey of the whole nebula to determine 

 the positions of all the stars visible in the 15-inch equatorial of the 

 Harvard Observatory. The catalogue contains the positions, relative to 

 ® Orionis, of 1101 stars, down to 'approximately the fourteenth magnitude. 

 The positions were for the most part determined by transits for right 

 ascension and mica scale readings for declination. In the cases of the 

 fainter stars, however, these methods were found impracticable and the 

 positions of a large number of these stars were determined from diagrams. 

 In some cases the diagrams were supplemented by one or two micrometer 

 measures, but for all the faint stars there is reason to suspect an appreciable 

 probable error. I was especially surprised to note the number of such 

 determinations in the Huyghenian region of the nebula. Out of 51 stars 

 assigned to this region by Professor E. S. Holden in his "Monograph on the 

 Central Positions of the ISTebula in Orion," the positions of eighteen were 

 determined by means of diagrams, while those of eight others were de- 

 termined from measures with reference to auxiliary stars, the positions of 

 which were no more accurately determined than those of the average of 

 the brighter stars. 



The positions of 47 of the bright stars in the nebula were determined 

 heliometrically by Bruno Meyermann at Groningen in 1905. Consider- 

 ing the large probable errors of former work on the faint stars, and 



