412 PLANETARY NEBULAE. SECT. XXXVI. 



found, the distance of this nebula would be determined. The 

 zodiacal light or lenticular shaped luminous haze surrounding the 

 sun which may be seen extending beyond the orbits of Mercury 

 and Yenus soon after sunset in the months of April and May, or 

 before dawn in November and December, seems to place our 

 luminary in the class of nebulous stars. The extensive and 

 delicate atmosphere of these nebulous stars assumes all degrees of 

 ellipticity, from the circular to the spindle-shaped ray, or almost 

 the right line. 



Planetary nebulas 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 vividness. 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 hardjy, if at all, visible to the naked 

 eye. From the uniformity of the discs of these planetary nebulas, 

 and their apparent want of condensation, he presumes that they 

 may be hollow shells emitting light from their surface only. 

 The southern hemisphere 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 colour, 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 nebula? or 

 double stellar nebulas of the utmost remoteness. Since Lord 

 Eosse's telescope has shown that five of the planetary nebula? are. 

 annular, some of those in the southern hemisphere may ulti- 

 mately be found to belong to the same class. 



Probably nine tenths at least of the nebulous contents of the 



