28 THOUGHTS ON NATURAL PHILOSOPHY. 



actually far beyond, is rendered overwhelmingly prob- 

 able from their position with reference to the cloud- 

 forms, as well as by certain relations that have been 

 shown to exist between their analyzed light and 

 that of the immediately surrounding nebula, in the 

 spectroscopic researches of Sir William Huggins. 



The diffuse character of the outlines of the nebula 

 renders it impossible to apply to it such delicate 

 measurements of direction as are necessary for the 

 determination of the parallax. For this reason its 

 distance cannot be directly investigated. The stars 

 of the trapezium have, however, shown no parallax ; 

 from this it becomes possible to assign roughly a 

 minimum limit beyond which they, and therefore in 

 all probability the nebula, must lie. Such distance 

 can scarcely be less than a million times that of the 

 Sun. To appear of its vast extent, even at this, the 

 most modest estimate, its glowing clouds must ex- 

 tend over such abysmal depths, that the whole of the 

 Solar System if plunged into it would become con- 

 temptible in its utter insignificance. 



The Nebula of Orion is a noble example of an 

 " irregular nebula." That of Andromeda, in its 

 regular ellipticity of outline, in the uniformity in the 

 central condensation of its light, and in the system of 

 elliptical rings by which it is enveloped, forms so 

 strong a contrast with it that it is difficult to regard 

 the two as objects belonging to the same class. 

 Other nebulae display a spiral structure ; others again 

 appear as fairly sharply defined planetary discs; 

 while the majority are to all appearance nothing more 

 than minute structureless clouds of flocculent light. 



