APPLIED TO THE HEAVENLY BODIES. 205 



certainly resolved into a close aggregation of bright points would give a spectrum 

 indicating gaseity. 



The observation with the prism of these objects is extremely difficult, on ac- 

 count of their great faintness. Besides this, it is only when the sky is very 

 clear and the moon is absent that the prismatic arrangement of their light is 

 even possible. During the last two years I have examined the spectra of more 

 than sixty nebulae and clusters. These may be divided into two great groups. 

 One group consists of the nebula which give a spectrum similar ro the one 1 

 have already described, or else of one or two only of the three bright lines. Of 

 the six objects examined, about one-third belong to the class of gaseous bodies. 

 The light from the remaining foi ty nebulte and clusters becomes spread out by 

 the prism into a spectrum which is apparently continuous. 



I will exhibit upon the screen a few of the more remarkable of the nebulce 

 which are gaseous in their constitution. 



This photograph is from a drawing by Lord Rosse of a small nebula in 

 Aquarius. (I. H. IV.) 



We have here a gaseous system which reminds the observer of Saturn and his 

 rings. The ring is seen edgeways. 



The three bright lines represent the spectrum into which the light of this ob- 

 ject is resolved by the prism. 



In this other nebula we find probably an analogous general form of structure. 

 In consequence of the nebula lying in a different position to us, its ring is seen, 

 not edgeways, but open on the flat. The spectrum consists of three bright 

 lines. 



The arrangement of the streams of light in the object now on the screen sug- 

 gests a spiral structure. This nebula is remarkable as the only one in which, 

 in addition to the three bright lines, a fourth line was also seen. 



The mo^t remarkable, and possibly the nearest to our system, of the nebulae 

 presenting a ring formation, is the well-known annular nebula in Lyra. The 

 spectrum consists of one bright line only. When the slit of the instrument 

 crosses the nebula, the line consists of two brighter portions, corresponding to 

 the sections of the ring. A much fainter line joins them, which shows that the 

 faint central portion of the nebula has a similar constitution. 



A nebula remarkable for its large extent and peculiar form is that known as 

 the dumb hell nebula. The spectrum of this nebula consists of one line only, 

 A prismatic examination of the light from different parts of this object shows 

 that it is throughout of a similar constitution. 



The most widely known, perhaps, of all the nebulse is the remarkable cloud- 

 like object in the sword-handle of Orion. 



This object is also gaseous. Its spectrum consists of three bright lines. Lord 

 Rosse informs me that the bluish-green matter of the nebula has not been 

 resolved by his telescope. In some parts, however, he sees a large number of 

 very minute red stars, which, though apparently connected with the irresolvable 

 matter of the nebula, are yet doubtless distinct from it. These stars v/ould be 

 too faint to furnish a visible spectrum, 



I now pass to some examples of the other great group of nebulaj and clusters. 



All the true clusters, which are resolved by the telescope into distinct bright 

 points, give a spectrum, which does not consist of separate bright lines, but is 

 apparently continuous in its light. There are many nehulce which furnish a 

 similar spectrum. 



I take, as an example of these nebuhe, the great nebula in Andromeda, which 

 is visible to the naked eye, and is not seldom mistaken for a comet. The 

 spectrum of this nebula, though apparently continuous, has some suggestive 

 peculiarities. The whole of the red and part of the orange are wanting. Be- 

 sides this character, the brighter parts of the spectrum have a very unequal and 

 mottled appearance. 



