SECT. IT. SPECTRA OF THE PLANETS. 157 



The colours of the stars are produced by vapours 

 existing in their atmospheres, one colour predominating 

 over the others, which are absorbed by the number of 

 dark lines. 



Messrs. Huggins and Miller obtained extraordinary 

 results from the examination of temporary and periodic 

 stars. Temporary stars suddenly shine forth with 

 great brilliancy and soon vanish or nearly vanish. A 

 temporary star which suddenly appeared on the night 

 of May 12, 1866, when examined with a spectroscope, 

 had two spectra, showing that its light emanated from 

 two distinct sources. One spectrum, analogous to that 

 of the sun, was formed by the light of an incandescent 

 solid or liquid photosphere, which suffered absorption 

 by the vapours of an envelope cooler than itself. The 

 second spectrum consisted of a few bright lines, indi- 

 cating that the light by which it was formed was 

 emitted by luminous gas : the position of some of the 

 lines denoted hydrogen ; whence the observers be- 

 lieved the phenomena to result from the burning of 

 hydrogen with some other element, and that the photo- 

 sphere was heated to incandescence by the resulting 

 temperature. 



The variation in the brightness of periodic stars has 

 by some been supposed to be due to an opaque body 

 periodically obscuring the light. Should that body be 

 surrounded by an atmosphere like our planet's, its pre- 

 sence would be revealed by the absence or presence of 

 additional lines of absorption in the spectrum of the star. 

 Now three lines determined in the spectrum of Betelgeux 

 were no longer found when the star arrived at its maxi- 

 mum of brightness, indicating it may be the presence 

 of an atmosphere round the opaque body. 



With regard to our own planets, Jupiter has lines in j 

 his spectrum which indicate the existence of an ab- ' 

 sorptive atmosphere ; one band indicates the presence 



