Jfir'" 



M. Trou 



E. S, Holden — On supposed changes in Nebula M. 17. 351 



the nebuloi as they would appear in a refractor. It is only in 

 this way that several drawings, made both by reflectors and 

 refractors can be satisfactorily and minutely compared. The 

 cuts of the present paper were reproduced from such photo- 

 graphic prints, and they are on a scale of one inch = 266""2 ; 

 in the present paper, however, all conclusions are drawn from an 

 examination of the original engravings, although references are 

 made to the cuts for convenience. 



My own notes on the physical aspect of this nebula are as 

 follows:— 



' The brighter portions of this nebula give evidence of resolv- 

 ular all the brighter parts of the " horseshoe," 

 branch extending from star 8 to star 71, seem 

 nder intense looking, to be just ready to break into small stars. 

 ' f small points of light were put down both by myself and by 

 rouvelot, but it was soon found that such a task was endless 

 and of far less importance than the correct delineation of the 

 nebulosity. For nearly all the details of the drawing M. Trouve- 

 lot is responsible, as time did not allow of that careful and inde- 

 pendent comparison which it is desirable should always be made ; 

 but it may be said briefly, that there are no conjectures laid down 

 in the original Everything is as it was seen. I confined my 

 attention principally to the space limited by stars, 1, 6, 9, 61, 63, 

 70, 20, 8, 3, 10, on account of the evidence from older drawings 

 that this portion has moved relatively to the stars ; and I can 

 vouch for its general accuracy. 



This portion is sufficiently accurate to found an argument for or 

 against variability upon. For example. I am sure that the 

 brightest mass of nebulosity follmcs star No. 1, as M. Trouvelot 

 has drawn it, and I am sure that the general direction of the dark 

 channels between stars 7 and 53 is correct in the drawing." — 

 [Sept. 21 to Oct. 2, 1875.] 



Trouvelot's star co-ordinates were obtained graphically from 

 the original sketch and are given in Table IV. 



II. Comparison of Star-positims. 

 The only relative star-places completely determined which 

 are of the highest accuracy are those of Lassell. "Eleven of 

 the principal stars were measured and the remainder laid down 

 some approximation to their pro- 

 tudes is preserved in the drawing. The num- 

 bers are generally, but not uniformly, in the descending order of 

 magnitude." Thie star-positions of Lamont are of high accuracy, 

 bat unfortunately only angles of position were measured in 

 most cases. Mason's work was done while he was an undergrad- 

 uate at Yale College, with a telescope of his own construction 

 mounted as an alt-azimuth, and in the intervals of his collegiate 



