526 WRIGHT— RECENT SPECTROSCOPIC OBSERVATIONS. 



of a star, assumed to be surrounded by envelopes of two kinds of 

 gas, one larger than the other. The vertical parallels represent the 

 slit of the spectroscope. To the right is the spectrum of the system 

 showing bright gaseous lines, and the continuous spectrum of the 

 star. The length of the bright lines is seen to offer a measure of 

 the extent of the gaseous distribution. The objects are so faint that 

 they can be observed only by photographs of long exposure, and in 

 making these it is necessary to keep the nebula at precisely the 



Uiagnm of Sficchtim of Shr Hurrountitd by 



Uiininnus C;tseous /I fmosjohere. 



Fig. 4. 



same position on the slit during the entire time, otherwise the record 

 would be hopelessly confused. This is a tedious, and sometimes 

 difficult task. 



Turning now from the diagram to a real subject, Fig. 5 records 

 the spectrum of the bright planetary in Andromeda. The fourth 

 nebular line is shorter than the others, which indicates that the pecu- 

 liar conditions that are favorable for its production obtain only com- 

 paratively close to the central star, in fact very largely within the 

 inner area indicated in the preceding sketch. The spectrum of the 

 nucleus is too faint to show well and is represented better in another 



