E. S. Holden— Proper Motion of the Trifid Nebula M!20. 455 
While, therefore, no definite conclusion can be reached (as 
Lassell’s drawings of nebule in general are of the highest 
must not only accept them as evidence, but as very positive 
and conclusive evidence), it is possible that some slight changes 
se brilliancy (apparently none of situation) may have oceurr red 
n the region 28, 17, 18, 26 during the ame forty years. 
Toning the previous order : 
G. According to Herschel: star 6 is immersed in A; Ma 
star 6 is immersed in A, but is 8 very near the edge; Lassell, for 
The evidence above seems to me to indicate some decided 
changes in the brightness [and position] of the edge of A near 
6. The first two and the last two authorities agree, and both 
differ from Lassell and from Langley. Accepting Lassell’s 
authority, some change has here taken place, and it appears 
that it Is connected witb the suspected changes F and Z. 
ones, but seem to confirm Lassell’s. 
J. The triangle defined by 40, 44, 21 is, according to Herschel: 
free from any marked nebulosity ; ; Mason, completely filled with 
tolerably bright nebulosity ; ae free from nebula (2d figure), 
free from nebula except A’ (1st figure); Trouvelot, contains a con- 
siderable amount of pretty bright nebulosity ; Holden, contains 
no nebulosity at all bright except A’. 
Here the various accounts do not agree. Positive evidence 
is more to be considered than negative, and this triangle in 
1837 was Rabel filled with nebula considerably bright. It 
is not so now. ‘T'rouvelot’s figure really agrees with Lassell’s 
and my own, if we correct a little distortion of his figure 
near this part. 
K and L. The point to be settled is, has the channel a remained 
unchanged? From Herschel’s figure we derive the following : 
The apex of A’ (which in his figure is continuous with B, and 
is indeed about the beishved portion of the nebula, quite different 
from now) is about 38 seconds of arc south of 1. The 4a of this 
apex is (approximately) —7*. The width of the channel a-at 
this point is 85 seconds. Bee the north shore of C in the 
R.A.—7* (from 1) is in Jd —12 
