E. S. Holden—On supposed changes in Nebula M. 17. 355 



It will be remembered that the seven drawings of this nebula 

 which we possess were made in the years 1833, 1837, 1839, 

 1862 and 1875 quite independently, by different observers and 

 different telescopes It is evident that in cases where these dif- 

 ferent drawings agree, there can be no doubt as to the existence 

 of the feature delineated. The non-existence of any prominent 

 feature not given is probable, although not certain. To ex- 

 amine the question proposed at the head of this section, it will 

 be advantageous to divide the drawings into three groups, the 

 first consisting of all figures made before 1840 (Herschel's, 

 Lamont's, Mason's), the second, of Lassell's fine delineation, 

 which is entitled to very great weight, and the third, of the two 

 drawings made by M. Trouvelot, one at Cambridge and the 

 other at Washington. It is well to recall the fact that Hers- 

 chel's two figures were made with his 20-foot reflector of 18^ 

 inches aperture; Lamont's with the Munich refractor of 11 

 inches ; Mason's with a 14-foot reflector of 12 inches ; Lassell's 

 with his 4-foot reflector; M. Trouvelot's first drawing with a 6^ 

 inch Merz refractor, and the Naval Observatory drawing with 

 the very perfect Clark refractor of 26 inches. 



To prove the existence of a change it is necessary and suffi- 

 cient to show that a prominent feature which the first group of 

 drawings give-!, is in a different position relative to the stars in 

 Lassell's drawing and that the motion thus shown is confirmed 

 and continued by the two figures of 1875, much greater weight 

 being given to the work of the larger instrument in 1875. It 

 must be remembered that with two instruments of equal light, 

 hardly more discrepancy in the positions of the ^/grA/er portions 

 of the nebula is to be expected than in the star-positions, for 

 these positions are determined by the stars themselves and can 

 be assigned with almost no error, in a nebula which contains 

 so many stars as the one under consideration. The fainter por- 

 tions may vary greatly from the smaller to the larger instru- 

 ments. No relative numerical weight can be assigned to the 

 various drawings, even if it were desirable to do this, but it 

 may be remarked that Mason's, Herschel's (1837), Lassell's and 

 that of the Naval Observatory are of the greatest authority. 

 Herschel's first drawing, he himself does not consider compar- 

 able in accuracy to his second : Lamont's is of great weight as 

 to the relative star- positions, but was not intended as anything 

 more than a "sketch," and Trouvelot's first figure while un- 

 doubtedly of high accuracy as far as his instrumental means 

 were satisfactory, is yet not strictly comparable with the work 

 of Herschel's and Lassell's reflectors, or to his own work with 

 the Naval Observatory refractor. 



