438 H. S. Holden—Proper Motion of the Trifid Nebula M. 20. 
The observations of Secchi* (1852-5) and of D’ Arrestt (1866, 
June 2) throw no light on the position of the triple star relative 
to the nebula. 
Later observations of the nebula are those of Lassell, Lang- 
ley, Trouvelot and myself. (It has also been observed at Mel- 
4-foot reflector, an instrument well suited to the purpose. They 
are published in the Memoirs of the Royal Astronomical Soci- 
ety, volumes xxxili and xxxvi. 
No verbal description of the nebula is given by Lassell, but 
the drawings are complete in themselves, and the evidence to 
_ be derived from them will be considered hereafter. The fol- 
lowing micrometric observations were made by Lassell. In 
order to present the nomenclature of the stars the Index-Map 
(Lassell’s) is given. 
Table [—LasseL’s Star-Posttions.t 
[The positions are referred to Star 1.] 
gusty Observed. Computed. 
Star —. Dp. 8. Aa. Ad, 
3 25° 467 270"°15 +117"44 | +243"°30 
4 333. 4 243-20 —110°16 +216°82 
5 ves. ee 8 | 215'37 +175°38 + 125°02 
q 116 49 181°21 +161°72 — 81°75 
8 311: 94 391°68 — + 259-03 
10 121 30 90:39 + TT07 
14 43 PEPE + 11°49 +176°80 
167 52 249°25 + 52°39 a 
21 235 40 129-17 —106°67 — 72°85 
* Mem. dell’ Oss. Coll. Romano, 1852-55, p. 89. 
{ Abhand. d. k. Sach. Gesell. d. Wissenschaften, voi. v, p. 343. 
Memoirs Royal Astronomical Society, vol. xxxvi, p. 49. 
