354 E. S. Holdm— On supposed changes in Nebula M. 17. 

 Table V. — Comparison of Lassbll, Mason, Hersohel and Tboutelot. 



No. 



Lassell- 

 Herschel, 



^Si: 



--',-, 



Remarks. 





Ac. 



Ai 



Aa. 1 AS. 







11 



6-2 



23-0 

 8-Of 



I 



^0-8 



^6-2 



1 



* Some doubt as to identity. 



mined are marked f, the rest are esti- 



" B^tiU be noted that Mason's resi- 

 duals are quite within his limits of 



It may be seen from table J II that the relative positions of 

 the principal star? of this group are the same during the period 

 of observation. Precession will cause clianges of relative posi- 

 tion far less than the errors of observation. 



III. Comparison of Htar-magnitudes. 



It will be remembered that Lassell's numbers represent 

 generally the order of brightness of the stars. Mason's magni- 

 tudes were reduced by himself so as to correspond to those of 

 Herschel, and this was done in Ms own thorough way, by 

 comparing his estimates of the brightness of stars in Nelmla 

 Cygni, with those previously given by Herschel. Lamont and 

 Trouvelot give no magnitudes. 



Table VI contains most of the information available on this 



Tablk VI.— Comparison op Star-magnitudes. 



Lassell's number, 



li2| . 



Ha ,ia|,o 



n|>. 



13i,.:2s!H .. 



Herschel'8 magnitude, 





nl„-u!,3lK 



4. 



12!,.^;12: .3 _ 



— »' jnu... 



r;i....|u..|u 



15112 



13J15 16,14^ 14 it) 



From thif 



i that the order of magnitude is as follows: 



2, 3, 5, 6, 



2, 3, 5, 6, 7, 13, 35, 8, II, 12, 73, 10, 15, 28; 

 Mason, 1, 2, 5, 6, 12, 3, 7, 13, 10, 35, 8, 73, II, 15, 28. 



A suspicion of variabilitv exists as to stars 12, 13, 10, 35, 8, 

 73, 11, 15,* 28, but 35, 8, 15and 73 are the only cases in which a 

 probahHity of variability exists and even in these cases it is 

 slight. If my identification of Lassell's 73 on the other draw- 

 ings is correct, there is a mistake in his magnitude. A toler- 

 ably bright star is very near its place. 



* See p. 359. 



