358 E. S. Holden — On supposed changes in Nebula M. 17. 



down by each observer no difficulty would be found in mak- 

 ing an exact comparison, and it will be found from an exam- 

 ination of the original engravings that the tracing of such a line 

 on them is a matter of some difficulty. Still, it is believed that 

 sufficient definiteness can be attained to show that on the whole 

 Trouvelot's Cambridge drawing is consistent with the con- 

 clusion above given. In particular, his star No. 1 is not at all 

 on the follotving edge of the nebulosity, but well within it, to- 

 wards the preceding side, thus totally differing from the ap- 

 pearances laid down by all earlier drawings. Again, he rep- 

 resents the space within the horseshoe and north-following star 

 No. 1, to be largely filled with nebulosity, quite consistently 

 with the Naval Observatory drawing, and utterly different from 

 the drawing of Lassell (see Fig. 6). This fact is of great im- 

 portance, since, if the nebulosity followed star No. 1, in 1862, 

 Lassell would have so represented it (as he did not) and as 

 Trouvelot has so drawn it, it is plain that an important change 

 must have occurred to render it possible for a six-inch aperture 

 to show nebulosity in 1875, in a space perfectly void of nebu- 

 losity to Lassell's great reflector in 1862. On the whole, then, 

 these drawings show that the western end of this nebula has moved 

 relatively to its contained stars from 1838 to 1862, and again from 

 1862 to 1875, and always in the same direction. 



I conceive that this is the best conclusion that can be drawn 

 from the ensemble of the drawings. If we confine our atten- 

 tion to the three best ones, viz: Herschel's (1887), Lassell's 

 (1862), and that of the Naval Observatory (1875), this conclu- 

 sion comes out with greater distinctness. There is only one 

 important feature in these three drawings which does not 

 strictly agree with this supposition, namely that star No. 1 is 

 in the same position with reference to the nebulosity in the 

 first two of these which were made at an interval of twenty- 

 five years and that they both differ from the last drawing made 

 thirteen years after Lassell's. In every other respect the agree- 

 ment is strictly with the above conclusion, and however much 

 weight I might have been inclined to give to this disagreement, 

 if the only data were those of the drawings, I cannot regard it 

 as final in the light of a most careful examination of the neb- 

 ula on the very fine night of March 21, 1876, when the various 

 drawings were compared with the heavens. I add from the 

 observing books a literal copy of my recorded observations on 

 that occasion. 



Mctract from Observing- Book. 



"1876. March 21. l^^-VJ\ Observer Holden, Recorder, 



D. P. Todd. 



Omega-Nebula. R A. 18" 12«' N. P. D. 106° 2. Magni- 

 fying power 175. 



