76 CELESTIAL PHENOMENA. 



enormous dimensions. New and highly ingenious considera- 

 tions ( 31 ) on the very different effect which distance produces 

 on the intensity of light of a disk of appreciable diameter, 

 and of a self-luminous point, render it not improbable that 

 the planetary nebulae are very remote nebulous stars, in which 

 the difference between the central star and the nebulous 

 envelope is no longer sensible even to our telescopic vision. 

 The magnificent zones of the southern celestial hemisphere, 

 between 50 and 80, are especially rich in nebulous stars, 

 and in unresolvable nebulas. Of the two Magellanic clouds 

 which revolve around the starless and desert southern pole, 

 the larger especially appears, by the most recent researches ( 32 ), 

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

 clusters and nebulae of different magnitudes, and of large 

 nebulous spaces not resolvable, which, producing a general 

 brightness of the field of view, form as it were the 

 background of the picture." The appearance of these 

 clouds, that of the brilliant constellation of the Shi]), 

 the milky way between the Scorpion, the Centaur, and 

 the Southern Cross, I may say, the graceful, and pictu- 

 resque aspect of the whole southern celestial hemisphere, 

 have left on my mind an ineffaceable impression. 



The zodiacal light which rises in a pyramidal form, and con- 

 stantly adorns the tropical nights with its mild radiance, is 

 either a vast rotating nebulous ringbetweentheEarthand Mars, 

 or, less probably, the outermost stratum of the solar atmos- 

 phere. Besides the luminous clouds and nebulae of definite 

 form, exact and always accordant observations indicate the ex- 

 istence and general distribution of an infinitely divided and ap- 

 parently non-luminous matter, which constitutes a resisting 

 medium, and manifests itself by diminishing the eccentricity, 



