154 LIGHT SCIENCE FOE LEISURE HOURS. 



coloured light is crossed by dark lines whose position 

 indicates the nature of the vapours which the light has 

 traversed. When the light comes from luminous 

 vapours the spectrum consists wholly of bright lines ; 

 and these have exactly the same position as the corre- 

 sponding dark lines which are seen when the same 

 vapours intercept light from an incandescent solid 

 mass. Lastly, when light is reflected from an opaque 

 substance, the spectrum is the same as that which 

 would be presented by the light before reflection, unless 

 the opaque substance is surrounded by vapours, in 

 which case the spectrum will be crossed by new dark 

 lines corresponding to the absorptive qualities exerted 

 by those particular vapours. 



We see then the wonderful qualities of the new 

 analysis. Applied to the sun and stars it has enabled 

 our physicists and astronomers to pronounce as con- 

 fidently that certain elements exist in these far distant 

 orbs, as the chemist can pronounce on the constitution 

 of substances submitted to his direct analysis. The 

 questions, or some of them, which have been at issue 

 respecting comets, will undoubtedly yield to the powers 

 of the spectroscope. The great want, at present, is a 

 brilliant comet to work upon. Donati's comet (1859), 

 or the great comet of 1861 would have served this 

 purpose admirably, but the first came in the very year 

 in which the principles of spectroscopic analysis were 

 first discovered; and the powers of the spectroscope 

 were only just beginning to be recognised when the 

 comet of 1861 made its brief visit to our northern 

 skies. 



