68 COSMOS. 



action of all the moving forces of the universe, the charm that 

 exercises the most powerful influence on the mind is derived 

 less from a knowledge of that which is, than from a perception 

 of that which ivill be, even though the latter be nothing more 

 than a new condition of a known material existence; for of 

 actual creation, of origin, the beginning of existence from 

 non-existence, we have no experience, and can therefore form 

 no conception. 



A comparison of the various causes influencing the develop- 

 ment manifested by the greater or less degree, of condensation 

 in the interior of nebulae, no less than a successive course of 

 direct observations have led to the belief that changes of 

 form have been recognised first in Andromeda, next in the 

 constellation Argo, and in the isolated filamentous portion of 

 the nebula in Orion. But want of uniformity in the power of 

 the instruments employed, different conditions of our atmo- 

 sphere, and other optical relations, render a part of the results 

 invalid as historical evidence. 



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 

 circular or slightly oval discs manifest in all their parts a 

 perfectly uniform degree of faint light. Nebulous stars are 

 not merely accidental bodies projected upon 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 

 remote 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 considera- 

 tions of the different influence exercised by distance* on the 

 intensity of light of a disc of appreciable diameter, and of a 

 single self-luminous point, render it not improbable, that the 

 planetary nebulas are very remote nebulous stars, in which 



* The optical considerations relative to the difference presented by a 

 Bingle luminous point, and by a disc subtending an appreciable angle, in 

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

 in Araa-o's Analyse des Travaux de Sir William Herschd, (Annuaire 

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



