E. S. Holdfn — On supposed changes in Nebula M. 17. 347 



In Huggins' paper already cited (p. 385) we find the follow 

 ing:— 



" Lord Oxmantown informs rae that in the observations of 

 this nebula at Birr Castle, there is no mention of resolvabilitv ; 

 and that ; the central part to the right [east ?] of star a [No. 

 2 ?] consists of bunches or patches of bright nebulosity with 

 fainter nebulosity intervening.' The spectrum of this nebula 

 indicates that it possesses a gaseous constitution. 



One bright line only was seen, occupying in the spectrum 

 apparently the same position as the brightest of the lines of 

 nitrogen. When the slit was made as narrow as the intensity 

 of the light would permit, this bright line was not so well 

 defined as the corresponding line in some of the other nebulae 

 under similar conditions of the slit, but remained nebulous at 

 the edges. 



When the brightest portion of the nebula containing the 

 nucleus or ' bright knot' was brought upon the slit, in addition 

 to the bright line a faint narrow continuous spectrun 

 The bright k _ ^ • 



condensed than it is' represeot* 

 Herschel." 



A very rapid method of drawing nebulae, is the following: 

 it yields to the first in the accuracy of the positions of the 

 stars, but it is probably even superior to it in facilities for the 

 correct representation of the nebula and stars considered as one 

 mass. A piece of glass ruled carefully into squares (see figs. 6 

 and 7) and this is placed in the focus of the eyepiece so as to be 

 plainly visible; the telescope is then directed upon the nebula, 

 and a clock-work motion is applied to the telescope so that it 

 follows the nebula accurately. Some one of the brighter stars 

 is chosen, and it is kept by means of the clock-work accurately 

 in the corner of one of the squares. A piece of paper ruled 

 into squares similar to those of the glass reticle is provided, and 

 ) observer dots down the various stars in and about the 



nebula. This may take two, three or four nights, according t< 



circumstances, but in all cases it requires much less time thai 



micrometric measurements of the brighter stars and the 



troublesome allineations required to fix the positions of the 

 smaller stars and it has the great advantage that the work can 

 be done in a perfectly dark field of view, whereas the micro- 

 metric measures demand the use of illuminated wires at least. 

 After the stars are inserted, the principal lines of the nebula 

 are put in, not only by the star groups, but also by the squares 

 themselves. For my own use I have had constructed two reti- 

 cles : one ruled in squares like those seen in figs 6 and 7, and 

 another in which the heavy-lined large squares (each containing 

 nine small squares, see fig. 6) are still present, but are subdi- 



