434 #. S. Holden—Proper Motion of the Trifia Nebula M. 20. 
Observations of Sir WILLIAM HERSCHEL 
1784, July 12: “Three nebule faintly joined form a triangle. 
In the middle is a double star. Very faint and of great extent.” 
—Phil. Trans., 1786, p. 494. 
1786, May 26: “A double star with extensive nebulosi sity of 
different intensity. About the double star is a black opening, re- 
sembling the nebula of Orion in miniature.”—Phil. Trans., 1789, 
. 247 
ll, ....: “Three nebule seem to join faintly together form- 
ing a kind of triangle; the middle of which is less nebulous, or 
perhaps free from nebulosity ; ; in the ameoe of the triangle is a 
double stur, ete.” —Phil. Trans., 1811, 
236 Sweep July 12, 1784: ce Between, 3 ers (10, 71, °49, 
V class) is a double star.”— Mem. R. A. 8., i, p. 
566 Sweep. May 26,1786: “ A double star yithin nebula IV, 
41.” — Mem. R. A. 8., 1, 
Observations of Sir JOHN HERSCHEL 
Se ee “Tn the nebula R. A. 17" 52™, N. P. D, 113° 1’ mn 
Sagittarius * * * * the idea of an absorption by the double star 
in its middle is very forcibly suggested. This nebula is broken 
into three parts, and the three lines of division meet in a vacancy, 
in the midst of which is situated the aoe star. Ti og a 
object has perhaps a proper motion.”--Mem. R. A. S., ii, p. 49 
8. 
os 
Fig. 1. HERSCHEL, 1833. 
1827....: “A double star placed exac tly in the central vacuity 
of a ine irregular nebula, which appears to have broken up into 
three portions by three rifts or cracks, extending from its ce miter 
to its circumference, pos ose directions meet at the double star.” 
—Mem. R. A. a: ili, p. 
et “The ie star Sh. 379 in the center of the trifid 
183 
nebula” . [Sweep 2 
