254: THE NEBULAE HYPOTHESIS. 



And, referring to this same order of objects, M. Arago 

 says : — " The forms of very large diffuse nebulae do 

 not appear to admit of definition ; they have bjo regular 

 outline." 



Xow this coexistence of largeness, irresolvability, 

 irregularity, and indefiniteness of outline, is extremely 

 significant. The fact that the largest nebulae are either 

 irresolvable or very diflicult to resolve, might have been 

 inferred a priori ; seeing that irresolvability, implying that 

 the aggregation of precipitated matter has gone on to but 

 a small extent, will be found in nebulae of wide diffusion. 

 Again, the irregularity of these large, irresolvable nebulae, 

 might also have been ex2:)ected ; seeing that their out- 

 lines, compared by Arago to " the fantastic figures which 

 characterize clouds carried away and tossed about by 

 violent and often contrary winds," are similarly charac- 

 teristic of a mass not yet gathered together by the 

 mutual attraction of its parts. And once more, the fact 

 that these large, irregular, irresolvable nebulae have 

 indefinite outlines — outlines that fade off insensibly into 

 surroundino- darkness — is one of like meaning^. 



Speaking generally (and of course differences of dis- 

 tance negative anything beyond an average statement), the 

 spiral nebulae are smaller than the irregular nebulae, and 

 more resolvable ; at the same time that they are not so 

 small as the regular nebulae, and not so resolvable. This is 

 as, according to the hypothesis, it should be. The degree of 

 condensation causing spiral movement, is a degree of con- 

 densation also implying masses of flocculi that are larger, 

 and therefore more visible, than those existing in an earliei 

 stage. Moreover, the forms of these spii'al nebulae are 

 quite in harmony with the explanation given. The curves 

 of luminous matter which they exhibit, are 7iot such aa 

 would be described by more or less discrete masses start- 

 in ;• irora a state of rest, and moving through a resisting 



