CELESTIAL PHENOMENA. 85 



Nebulous stars must not be confounded either with irregu- 

 larly-shaped nebulous spots, properly so called, whose separate 

 parts have an unequal degree of brightness (and which may, 

 perhaps, become concentrated into stars as their circumference 

 contracts), nor with the so-called planetary nebulae, whose cir- 

 cular or slightly oval disks manifest in all their parts a per- 

 fectly uniform degree of faint light. Nebulous stars are not 

 merely accidental bodies projected upr i a nebulous ground, 

 but are a part of the nebulous matter constituting one mass 

 with the body which it surrounds. The not unfrequently con- 

 siderable magnitude of their apparent diameter, and the re- 

 mote distance from which they are revealed to us, show that 

 both the planetary nebula? and the nebulous stars must be of 

 enormous dimensions. New and ingenious considerations of 

 the different influence exercised by distance*" on the intensity 

 of light of a disk of appreciable diameter, and of a single self- 

 luminous point, render it not improbable that the planetary 

 nebulae are very remote nebulous stars, in which the differ- 

 ence between the central body and the surrounding nebulous 

 covering can no longer be detected by our telescopic instru- 

 ments. 



The magnificent zones of the southern heavens, between 

 50° and 80°, are especially rich in nebulous stars, and in com- 

 pressed unresolvable nebulae. The larger of the two Magel- 

 lanic clouds, which circle round the starless, desert pole of the 

 south, appears, according to the most recent researches,! as 

 " a collection of clusters of stars, composed of globular clusters 

 and nebulae of different magnitude, and of large nebulous spots 



* The optical considerations relative to the difference presented by 

 a single luminous point, and by a disk subtending an appreciable angle, 

 in which the intensity of light is constant at every distance, are explain 

 ed in Arago's Analyse des Travaux de Sir William Herschel (Annuaire 

 du Bureau des Long., 1842, p. 410-412, and 441). 



t The two Magellanic clouds, Nubecula major and Nubecula minor, 

 are very remarkable objects. The larger of the two is an accumulated 

 mass of stars, and consists of clusters of stars of irregular form, either 

 conical masses or nebulae of diflferent magnitudes and degrees of con 

 densation. This is interspersed with nebulous spots, not resolvable 

 into stars, but which are probably star dust, appearing only as a general 

 radia^nce upon the telescopic field of a twenty -feet reflector, and form- 

 ing a luminous ground on which other objects of striking and inde- 

 scribable form are scattered. In no other portion of the heavens are 

 so many nebulous and ste.lar masses thronged together in an equally 

 small space. Nubecula minor is much less beautiful, has more unre- 

 solvable nebulous light, while the stellar masses are fewer and fainter 

 in intensity. — (From a letter of Sir John Herschel, Feldhuysen, Cap6 

 of Good Hope, 13th June, 1836.) 



