NEBULAE. 187 



gested the nebular hypothesis. " A most singular phenomenon," he remarked of one of 

 these objects, of which a representation is annexed, as the side view, marked with a white 

 line, " a star of about the eighth magnitude, with a faint luminous atmosphere, of a 

 circular form, and about 3' in diameter. The star is perfectly in the centre, and the 

 atmosphere so diluted, faint, and equal throughout, that there can be no surmise of its 

 consisting of stars." Stellar nebulae occur in great numbers. The spherical form is not, 

 however, invariable. Some are oblong and spindle-shaped ; others are double, treble, or 

 quadruple ; but the great majority are circular individuals ; and all show decided com- 

 pression towards a central point. Specimens of these varieties are shown in the side view, 



a slight inspection of which is suf- 

 ficient to vindicate the soundness 

 of Herschel's view of their con- 

 struction. " The idea," says he, 

 11 of condensation occurs so natu- 

 rally to us when we see a gradual 

 increase of light, that we can hardly 

 find a more intelligible mode of 

 expressing ourselves than by call- 

 ing it condensed. Instead of en- 

 quiring after the nature or the 

 cause of the condensation of nebu- 

 lous matter, it would, indeed, be 

 sufficient to call it merely a con- 

 densing principle ; but, since we 

 are already acquainted with the 

 centripetal force of attraction, 

 which gives a globular figure to 

 planets, keeps them from flying 

 out of their orbits in tangents, and 

 makes one star revolve around 

 another, why should we not look 

 up to the universal gravitation of 

 matter as the cause of every con- 

 densation, accumulation, compres- 

 sion, and concentration of the 

 nebulous matter?" There are many 

 examples, also, of faint fan-shaped 

 nebulosities being attached to stars, 

 and nebulous streams extending 

 from one star to another, and the designation of Planetary Nebula is given to other spe- 

 cimens in which there is a plain appearance of structure, but without any apparent 

 condensation. These objects exhibit an equally diffused light, with disks frequently 

 sharp and well-defined, and some of them rival in brightness the actual planets, 

 having minute attendants, like satellites. A fine example is in Aquarius, which has an 

 apparent sensible diameter of 20". Sir John Herschel computes that, supposing this 

 object to be as far from us as the stars, it must surpass in magnitude the area circum- 

 scribed by Uranus. He concludes, however, that if it be a solid body of a solar nature, 

 its intrinsic splendour is greatly inferior to that of the sun ; because a circular portion of 

 the sun's disk subtending an angle of 20" would shine with a light equal to that of a 

 hundred full moons, whereas the object in question is not discernible without a telescope, 



