456 E. 8. Holden—Proper Motion of the Trifid Nebula M. 20. 
From Mason’s figures (assisted here by the measured posi- 
tions of stars 4 and 6 of his 1 list) we find the south shore of B 
in 46 —51”; and the 46 of the extreme northern limit of .C 
187”. An acquaintance with Mason’s memoir is necessary in 
order to understand the great dependence to be placed on his 
wor 
In Lassell’s figure we find star 21 nearly on the northern 
limit of C, and hence in the (measured) 46 —73’"9. In La 
sell’s fioures the part A’ is completely detached from B by a 
wide, black opening. In Trouvelot’s drawing A’ is continuous 
with B, and in my own sketches A’ is indeed separated from 
B, but by a narrow and not very obvious channel. In respect 
to the southern parts of B we have then the following succes- 
sive results. 
From my own observations the contour of the north shore of 
C near stars 21, 36, 37, 38 is similar to Lassell’s, and the 46 of 
21 which gives s the 46 of this north shore is —65”. Collecting 
the various authorities— 
} Herschel: 46 of north shore of C = ‘ 
Mason: J6 of north shore of C ale (measured. ) 
Lassell : As of north shore of C’-—=— 73”. ibs 
Trouvelot: 46 of north shore of C —=— 60’. 
Holden: 46 of north shore of C =— 65’. “ip 
The last three authorities may be said to agree: the first 
two agree in placing the north shore of C 1’ or more further 
south. The amount by which it is further to the south is prob- 
' ably best given by Mason’s figures, for reasons already cited 
here, and spoken of in general by Herschel himself (doc. cit, 
. 11, foot note. 
If there had been no stars (Mason’s 4 and 6) near the area in 
question, this conclusion would not be so definite as it now 
appears to be. In fact, the positions of these stars were fixed 
and then the nebalosity was drawn about them. It is certainly 
not about these stars n 
The space A’ about 7° oe 1 and between the parallels of 
4d=0" and 46=-—38" is, according to Herschel, about the 
brightest portion of the nebula and S ainuaes with the rest of 
B; Mason, nearly equal to the brightest parts of the nebula, and 
continuous with the rest of B; Lassell, totally black ; Trouvelot, 
tld bright, and part of B; Holden, bright, but separ ated from 
y a narrow channel. 
n this respect, regarding all the drawings as of equal or 
ae equal accuracy, there has undoubtedly been a decided 
change. 
