26 POPULAR SCIEJSGE MONTHLY. 



them naturally tend to collect in clusters. It is, however, a curious 

 fact that, so far as yet been noticed, the large, diffused nebulas 

 which we have mentioned are more numerous in or near the Milky 

 Way. If this tendency is established it will mark a curious distinction 

 between them and the smaller nebulas. 



The most interesting question connected with these objects is that 

 of their physical constitution. When, about 1866, the spectroscope 

 was applied to astronomical investigation by Huggins and Secchi, these 

 two observers found independently that the light of the great nebula 

 of Orion formed a spectrum of bright lines, thus showing the object to 

 be gaseous. This was soon found to be true of the nebulae generally. 

 There is, however, a very curious exception in the case of the great 

 nebula of Andromedas. This object gives a more or less continuous 

 spectrum. Why this is it is difficult to say. 



Beyond the general fact that the light of a nebula does not come 

 from solid matter, but from matter of a gaseous or other attenuated 

 form, we have no certain knowledge of the physical constitution of these 

 bodies. Certain features of their constitution can, however, be estab- 

 lished with a fair approach to accuracy. Not only the spectroscopic 

 evidence of bright lines, but the aspect of the objects themselves, show 

 that they are transparent through and through. This is remarkable 

 when taken in connection with their inconceivable size. Leaving out 

 the large diffused nebulae which we have mentioned, these objects are 

 frequently several minutes in diameter. Of their distance we know 

 nothing, except that they are probably situated in the distant stellar 

 regions. Their parallax can be but a small fraction of a second. We 

 shall probably err greatly in excess if we assume that it varies between 

 one-hundredth and one-tenth of a second. To assign this parallax is 

 the same thing as saying that at the distance of the nebulas the dimen- 

 sions of the earth's orbit would show a diameter which might range be- 

 tween one-fiftieth and one-fifth of a second, while that of Neptune 

 would be more or less than one second. Great numbers of these ob- 

 jects are, therefore, thousands of times the dimensions of the earth's 

 orbit, and probably most of them are thousands of times the dimen- 

 sions of the whole solar system. That they should be completely 

 transparent through such enormous dimensions shows their extreme 

 tenuity. Were our solar system placed in the midst of one of them, 

 it is probable that we should not be able to find any evidence of its 

 existence. 



A form of matter so different from any that can be found or pro- 

 duced on the surface of the earth can hardly be explained by our ordi- 

 nary views of matter. A theory has, however, been propounded by Sir 

 Norman Lockyer, so ingenious as to be worthy at least of mention. It 

 is that these objects are vast collections of meteorites in rapid motion 



