NOTES ON NEBULAE. 131 



for by the belief that the Great Nebula was either very 

 much nearer or very much farther than the multiple star, 

 and that they chanced to lie in the same line of sight, 

 and had no other connection. But to me it appears that 

 this view is quite at variance with every reasonable pro- 

 bability; that the most wondrous multiple star should 

 have happened to lie in line with the very centre of the 

 most wondrous nebula, would have been a coincidence, 

 against the occurrence of which the probabilities were 

 almost infinite. There can scarcely be any doubt that the 

 multiple star and the Great Nebula are part of the same 

 system, and that the star is, in truth, placed in the middle 

 of the nebula, as it actually appears to be. 



And now as to the composition of this mysterious object. 

 Here, indeed, the terrestrial analogies seem to render us 

 but little assistance. While we were discoursing about 

 the moon, we could appeal to the volcanoes, both active 

 and extinct, on the globe, as offering some clues to the 

 nature of the lunar craters. So also when we were 

 speaking of Saturn we were able to derive some assistance 

 in our attempt to understand the appearance of its globe 

 from the analogy of terrestrial clouds. No similar re- 

 source is open when we study the nebulae; we look in 

 vain for natural phenomena on this globe which shall 

 render the needful clue. 



The word nebula means, of course, a little cloud, but 

 the expression is apt to be a misleading one. In a sense 

 no doubt they are little, inasmuch as the patch of the sky 

 which a nebula covers would be small compared with one 

 of our ordinary clouds. Indeed, a nebula which covered 

 as large an apparent part of the sky as the size of the 



