356 E. S. Holden—On supposed changes in Nehula M'. 17. . 



In Table YII, I have thrown into a convenient form the 

 evidence for or against a motion of the " horseshoe " with 

 reference to its contained stars. It will be remembered that 

 so far as our evidence goes (see Tables III and V) the rela- 

 tive position of the stars themselves has not changed; as 

 the agreement of the careful measures of Lament and Lassell 

 (whose observations are separated by 25 years) is quite as good 

 as can be expected. In Table V none of the residuals are 

 greater than the limits of error supposed by the various ob- 

 servers to exist in their own work ; and most of them are less. 

 It is moreover, plain that if the star-positions had changed 

 while the nebula also moved, even this fact would not effect the 

 question as to whether the line joining two stars, say 10 and 

 11, was inside or outside of the nebulosity. It is therefore in 

 this way that I have presented the evidence concerning the 

 motion of the " horseshoe " relative to the principal stars. 

 That is, I take a certain line, as that one joining stars No. 10 

 and 11 and terminated by them, and arranging the drawings 

 chronologically, I enquire how this line is situated with regard 

 to the nebula '^in the various delineations Is it all inside, or 

 partly outside ? If partly outside, what fraction of its length 

 is outside ? The single exception to this method is that of star 

 11, whose remarkable situation in the various drawings of the 

 nebula first led me to suspect a change. 



The number of independent proofs of the change, is not quite 

 so great as the number of columns in Table VII, since most of 

 the evidence rests upon the positions of stars i, 34, 35, 2, 10, 



It will be seen that not only is the change progressive from 

 group I to group II and from this to group III but that in 

 general it is even progressive from drawings 1 to 7. 



The exception to this general progression is fig. 6, made by 

 M. Trouvelot with his 6| telescope shortly before the drawing 

 No. 7. It will be observed that in many particulars his de- 

 scription corresponds to that given by the figures 1 to 4 ; which 

 might indicate that the differences observed are only such as 

 might be expected from the employment of different telescopes 

 by different observers. This explanation I do not believe to 

 be the correct one, for the reason that, first, no explanation is 

 thus attained of the large and consistent differences between 



mum brightness in the early figures, and in that made by M. 

 Trouvelot at Cambridge, shows that so far as the evidence goes, 

 the line of maximum brightness in the western half of the horse- 

 shoe, is now further to the east than it was in 1834-9. It is 

 evident that if the line of maximum brightness was plainly laid 



