220 



Mr. A. Le Sueur on the 



[Feb. 17, 



a long chapter of weary heart-breaking watchings, with an occasional half 

 hour's work. 



77 Argo was the first object observed for purpose of delineation ; after 

 the first night's work little (and that by snatches) was done towards it, a 

 new inroad of workmen and a long course of extremely unfavourable weather 

 having carried the nebula out of convenient reach. The search, which was 

 reluctantly given up, will, however, be again soon resumed. 



I enclose two sketches, 4403 and 3570, of the 18G4 catalogue. 



4403. The horseshoe nebula is a grand object, conspicuous and with 

 shape even in the finder (Plate I.). In the sketch the principal stars are laid 

 down from measured position-angles about different centres ; they are not 

 as accurate as I could wish, and will be reobserved differently under better 

 conditions ; in no case, however, can there be sufficient error to influence in 

 any material degree the configurations of the nebula or the smaller stars 

 sketched in by eye.] 



It will be seen that the sketch contains considerably more detail than 

 the corresponding figure in Herschel's Catalogue ; there appears, however, 

 to be no marked difference (with perhaps one exception) which may 4 not be 

 accounted for by the difference of aperture used. 



The exception to which I allude is the presence of a small but conspicu- 

 ous double star at the s. p. angle of the knot which lies between the and 

 the bright streak ; the experiment has not been tried of cutting down the 

 aperture to approximate to an 18-inch Herschelian, but the intrinsic 

 brightness of the principal star, and the presence in the C. G. H. of stars 

 not more bright (No. 3 of Herschel's catalogue is certainly less bright) go 

 far to show, without this experiment, that the star did not exist as such with 

 its present brilliancy at the time of theC. G. H. and P. T. 33 observations 

 ... I have not seen Mr. Mason's drawing, but look forward with much in- 

 terest to examining: it and his remarks thereon. 



o 



The important position of the star, and the careful scrutiny which the 

 knot and its neighbourhood must have repeatedly undergone, forbid the 

 assumption that it was simply overlooked by Sir John Herschel. 



The star /3 (I keep to Sir John Herschel's numbers and letters) is con- 

 spicuously and beautifully double, the companion of considerable brilliancy, 

 about 15 mag. ; with its present brilliancy and elongation it should, I think, 

 be within reach of an 18-inch. 



The knot is what I presume should be called resolvable ; the appearance 

 is sparkling, though no discrete stars can be seen, except perhaps a second 

 faint one, which is suspected at the s. f. angle. Part of the streak near to 

 the knot is also sparkling, but not in so marked a manner ; the other 

 portions appear of the ordinary milky nebulosity. 



The fainter nebulosity (S) of thebright streak pretty well marks out the 

 borders of the almost vacuous lane which leads up to and past the knot. 

 On receding from the lane it becomes very faint : nor is this faintness uni- 

 form ; but the appearances are so fugitive that, after repeated and painful 



