OBSERVATIONS OF HERSCHEL AND LORD ROSSE. 



90. The star w Centauri presents one of the most striking ex- 

 amples of the class of large diffused clusters. It is nearly round, 

 and has an apparent diameter equal to two-thirds of that of the 

 moon. This remarkable object was included in Mr. Dunlop's 

 catalogue ("Phil. Trans." 1828) ; but it is from the observations 

 of Sir John Herschel, at the Cape, that the knowledge of its 

 splendid character is derived. That astronomer pronounces it, 

 beyond all comparison, the richest and largest object of the kind 

 in the heavens. The stars composing it are literally innumerable ; 

 and as their collective light affects the eye hardly more than that 

 of a star of the fifth magnitude, the minuteness of each of them 

 may be imagined. The apparent magnitude of this object is such 

 that, when it was concentric with the field of Sir J. Herschel's 

 20-feet telescope, the straggling stars at the edges were beyond 

 the limit of the field. In stating that the diameter is two-thirds 

 of the moon's disk, it must be understood to apply to the diameter 

 of the condensed cluster, and not to include the straggling stars at 

 the edges. When the centre of the cluster was brought to the 

 edge of the field, the outer stars extended fully half a radius 

 beyond the middle of it.* 



The appearance of this magnificent object resembles that shown 

 in fig. 18, only that the stars are much more densely crowded 

 together, and the outline more circular, indicating a pretty exact 

 globe as the real form of the mass. 



91. The great nebula in Orion. The position of this extraordi- 

 nary object is in the sword-handle of the figure which forms the 

 constellation of Orion. It consists of irregular cloud-shaped 

 nebulous patches, extending over a surface about 40' square ; that 

 is, one whose apparent breadth and height exceed the apparent 

 diameter of the moon by about one-third, and whose superficial 

 magnitude is, therefore, rather more than twice that of the moon's 

 disk. Drawings of this nebula have been made by several ob- 

 servers, and engravings of them have been already published in 

 various works. 



In fig. 61 is given a representation of the central part of this 

 object. The portion here represented has a height and breadth 

 about one-sixth less than the diameter of the moon. An engrav- 

 ing upon a very large scale, of the entire extent of the nebula, 

 with an indication of the various stars which serve as a sort of 

 landmarks to it, may be seen by roference to Sir J. Herschel's 

 " Cape Observations," accompanied by the interesting details of 

 his observations upon it. 



Sir J. Herschel describes the brightest portion of this nebula as 



* Cape Observations, p, 21. 



45 



