16 



ProccedmijH of the Rotjal Irish Academi/. 



The following table contains the results of my meusares, giviiij; 

 first the description of the object, then its coordinates x and y witli 

 regard to the " central star," and finally the ^ and 77, the standard 

 coordinates, and in the last column the number of occasions on which 

 each coordinate was measured. It should be remembered that x and 

 y are only approximate values, both as regards scale and orientation 

 of the axes ; but I have thought it useful to give them here as a check 

 on the " standard coordinates " computed from them, and should better 

 ]daces of the four reference stars hereafter become available, it 

 would be easy to construct a table for coiTecting the values of ^ and // 

 here given. The latter are apparent coordinates, and must be 

 corrected for refraction and aberration by the table on p. 10, before 

 being compared with the results of other measures. The general view 

 of the nebula (Plate I.) is an enlargement of a plate taken on the 

 16th January, 1903, exposure 90". It extends much farther than my 

 measures do, and gives an excellent idea of the nebula, although the 

 spiral form is much more clearly seen on the original negatives. Thc' 

 skeleton chart (Plate II.) has been constructed from the measures and 

 descriptions, and was afterwards compared with the plate as seen in 

 the viewing microscojie of the measuring machine, as well as with a 

 second plate taken by Dr. Roberts on the loth December, 1900, with 

 an exposure of 90™. jS'either in the descriptions nor on the chart 

 was it found possible to distinguish sharply between small stars and 

 nebulous "knots" or condensations, except whenever the words 

 "large" or "small," or " knots," occur, which were only used of 

 unmistakable nelmU'e. 



[Catalogue. 



