238 SPECIAL RESULTS IN THE UEANOLOGICAL 



of which the photosphere has two or three minutes 

 diameter ( 412 ). 



A class of nebulae very different from those which we have 

 been describing, and which have always at least a faintly 

 marked outline, consists of the larger nebulous masses of 

 irregular form. These are characterised by very various and 

 unsymmetrical shapes, as well as very imperfectly denned and 

 confused outlines. They are mysterious phenomena "sui 

 generis," and are what have principally given occasion to 

 the opinions which have prevailed respecting the existence 

 of cosmical cloud, and of self-luminous nebulous matter 

 dispersed through the celestial regions and similar to the 

 substratum of the zodiacal light. A most striking contrast 

 is presented by viewing some of the irregular or amorphous 

 nebulae which cover several square degrees of the surface of 

 the heavens, in comparison with the smallest of all the re- 

 gular isolated oval nebulae with which we are acquainted, 

 i. e. the one situated between the constellations of Ara and 

 Pavo in the southern hemisphere, and which has a luminous 

 intensity equal to that of a telescopic star of the 14th mag- 

 nitude ( 113 ). No two of the unsymmetrical, diffused ne- 

 bulous masses resemble each other, " but," adds Sir John 

 Herschel, after many years of observation, " they have one 

 important character in common ; they are all situated in or 

 very near the borders of the Milky Way" ; and may be 

 "regarded as outlying, distant, and as it were detached 

 fragments of the great stratum of the Galaxy" ( 414 ). On 

 the other hand, the regular symmetrical and usually well- 

 defined small nebulae are partly scattered generally over the 

 heavens, and partly crowded into particular regions remote 



