510 



REVERSION OF BRIGHT LINES. 



lines which are artificially produced appear perfectly 

 black when the luminous part of the spectrum is very 

 bright ; but they only appear black by their contrast 

 with the bright vicinity ; in reality they are brighter 

 than the bright lines produced by incandescent vapour 

 when observed by a spectroscope, as may be proved 

 by experiments, which, however, require large and com- 

 plicated spectroscopes. The cause of this diminution 

 of light in those lines which appear dark when com- 

 pared with the remainder of the spectrum, is the 

 property of incandescent vapours, that they are not 

 equally transparent for all colours ; incandescent vapours 

 transmit light of all colours completely, except light of 

 the special colour or colours which they emit them- 

 selves ; these particular rays when emitted by any body 

 whatever are partly absorbed in their passage through 

 the glowing vapour. 



The yellow vapour of incandescent sodium compounds, 

 for instance, is nearly opaque for yellow rays of the same 



FIG. 281 (an.proj. \ real size}. 



kind as those which it emits itself, while it is perfectly 

 transparent for every other colour. This may be seen 

 by placing a small paraffin lamp behind the slit, and a 



