DUMB-BELL NEBULA. 



peculiar form. Sir J. Herschel considers that the most remark- 

 able circumstance attending it is the faint nebulosity which fills 

 up the lateral concavities of its form, and in fact converts them 

 into protuberances, so as to render the whole outline a regular 

 ellipse, having for its shorter axis the common axis of the two 

 bright masses. If it be regarded as a mass in rotation, it is 

 around this shorter axis it must revolve. Tn that case, he 

 considers that its real form must be that of an oblate spheroid ; 

 and as it does not follow that the brightest portions must be of 

 necessity the densest, this supposition would not be incompatible 

 with dynamical laws, at least supposing its parts to be capable of 

 exerting pressure on each other. But if it consist of distinct 

 stars this cannot be admitted, and we must then have recourse to 

 other suppositions to account for the maintenance of its form. Sir 

 John Herschel, it will be observed, failed to resolve this nebula. 



Fig. 38 is the same object as shown by the telescope of Lord 

 Rosse, three- feet aperture, twenty-seven feet focal length, 



Fig. 39 is the same object as shown with the great telescope of 

 Lord Rosse, six feet aperture, fifty-three feet focal length. 



The difference between these twa representations and that 

 given by Sir John Herschel of the same object, will illustrate in 

 a very striking manner the observations already made on the 

 effects of different magnifying and defining powers upon the 

 appearance of the object under examination. These three figures 

 could scarcely be conceived to be representations of the same 

 object. 



To explain the difference observable between the drawing 

 fig. 38, made with the smaller telescope, and the drawing fig. 39, 

 made with the larger instrument, Lord Rosse observes, that while 

 the application of a high magnifying power brings out minute 

 stars not visible with lower powers, it completely extinguishes 

 nebulosity which the lower powers 



render visible. The optical reason for <)g ' 



this will be easily perceived ; the cir- 

 cumstance was nevertheless overlooked 

 when the observations were made from 

 which the drawing fig. 38 was taken. 

 Only one magnifying power, and that a 

 very high one, was used on that occa- 

 sion, the consequence of which was that, 

 although the two knobs of the dumb- 

 bell were more fully resolved, the 

 nebulous matter filling the inter- 

 mediate space, which Herschel considered to be the most 

 remarkable feature of this nebula, was entirely extinguished in 



37 



