SECT, xxxvii.] PLANETARY NEBULA. 437 



Mercury and Venus, soon after sunset in the months of 

 April and May, seems to place our sun among the class of 

 stellar nebulae. 



Planetary nebulae have exactly the appearance of planets 

 with round or oval discs, sometimes sharply terminated, at 

 other times hazy and ill-defined. Their surface, which is 

 blue or blueish white, is equable or slightly mottled, and 

 their light occasionally rivals that of the planets in vivid- 

 ness. They are generally attended by minute stars, which 

 give the idea of accompanying satellites. These nebulae are 

 of enormous dimensions. One near y Aquarii has a sensible 

 diameter of about twenty seconds, and another presents a 

 diameter of twelve. Sir John Herschel has computed that, 

 if these objects be as far from us as the stars, their real 

 magnitude, on the lowest estimation, must be such as would 

 fill the orbit of Uranus. He concludes that, if they be solid 

 bodies of a solar nature, their intrinsic splendour must be 

 far inferior to that of the sun, because a circular portion of 

 the sun's disc subtending an angle of twenty seconds would 

 give a light equal to that of a hundred full moons ; while, 

 on the contrary, the objects in question are hardly, if at all, 

 visible to the naked eye. From the uniformity of the discs 

 of these planetary nebulae, and their apparent want of con- 

 densation, he presumes that they may be hollow shells 

 emitting light from their surface only. The southern hemi- 

 sphere is very rich in them, where twenty-eight or twenty- 

 nine have been discovered, some in the midst of a cluster of 

 stars, with which they form a beautiful contrast. Three are 

 of a decided blue clour, one Prussian blue, or Verditer green, 

 the other two of a bright sky blue, of great beauty and 

 delicacy. One seems to belong to the class of double nebulae 

 or double stellar nebulae of the utmost remoteness. 



Multitudes of nebulae appear to the unassisted eye, or 

 are seen with ordinary telescopes, like round comets without 

 tails; but, when viewed with powerful instruments, they 

 convey the idea of a globular space, insulated in the heavens 



