360 M S. Holden—On supposed changes in Nehula M. 17. 



shoe with reference to its contained stars acquires new weight. 

 The eastern half of the horseshoe, or at least that portion of it 

 north of stars 2 and 7 shows on the contrary no evidence of 



The observed changes in the drawings may be best accounted 

 for by supposing a bodily shifting of the whole of the horseshoe 

 in a plane nearly perpendicular to the line of sight, and on a 

 pivot situated somewhere in the region of star No. 8, though, 

 of course, it is not supposed that this is a real explanation of 

 the physical changes. 



A careful study of the evidence relating to the Messierian 

 streak indicates no motion with reference to the contained stars. 

 Graphic methods lead to the angles of position of this portion 

 given below, with which I have incorporated the results of 

 measures by D' Arrest and Schoenfeld, 



Herschel, 1833: p=119°. f Schoenfeld, 1862 : p=112°, 



Herschel, 1837: p— 119°. mean of two, 115°,110^. 



Lament, 1837: p=113°? Lassell, 1863 : ? 



Mason, 1839: p=115°. Trouvelot, 1875 : p=113°. 



*D'Arrest, 1855 : p=122", Naval Observatory, 1875: p=ll 9°. 

 mean of two, 128°, 1 1 6°. 



To sum up : — Tables III and V show that the stars have re- 

 mained in their relative positions from 1837 to 1875 ; and a 

 consideration of the drawings, whether taken as a whole or con- 

 sidered according to their relative importance, shows that the 

 horseshoe has moved with reference to the stars while the 

 Messierian streak has not moved, and that therefore we have evi- 

 dences of a change going on in this nebula. 



This may be a veritable change in the structure of the nebula 

 itself such as was suspected by Schroeter, confirmed by Otto v. 

 Struve and again confirmed by myself in the Nebula of Orion, 

 or it may be the bodily shifting of the whole nebula in space in 

 some plane inclined to the line of sight. 



A remarkable instance of a proper motion of this latter kind 

 is that of the Trijid Nebula G. C. 4355, which has moved since 

 1833 so that the remarkable triple-star which was then quite 

 clear from the nebulosity in a dark space formed by the junc- 

 tion of the three dark channels, is now bv the evidence of Las- 

 sell (1868) Winlock and Trouvelot (1874) and myself (1875) well 

 involved, the motion being confirmed by Herschel's drawing at 

 the Cape of Good Hope (1887) and Mason's of about the same 



The importance of the theoretical conclusions as to the con- 

 stitution and distance of the nebulae, to be derived from the 

 first well -authenticated instance of the variation in form of any 

 one nebula, have seemed to me to justify the discussion of the 



* Abhand. d. K. Sach. Ac. d. Wissenscbaften, Bd. v. 



f Ast. Beob. Mannheim. Zweite Abth., 1815. 



