REVERSION OF BRIGHT LINES. 509 



contain a particularly bright red line ; but instead of an 

 increase in brightness of a narrow portion of the spec- 

 trum, we see the red lithium line to be actually extin- 

 guished or a dark line appear in its place. 



To explain this, we must consider in the first place, 

 that neither the dark lines in the solar spectrum, nor 

 the dark lithium line produced artificially by reversion, 

 are absolutely black ; they are only less luminous than 

 their vicinity. The human eye is very susceptible to 

 effects of contrast ; a moderately bright object appears 

 much brighter when around it there is comparative ab- 

 sence of light ; and if its neighbourhood is brighter than 

 the object itself, it will appear much darker than it 

 would appear if the object and its vicinity were equally 

 bright. Thus if gradually more light is thrown into 

 the neighbourhood of an object which is moderately 

 bright, the object will appear to become obscured. 



This may be shown by Rumford's photometer. Let 

 a burning candle and a lamp be so placed that the 

 shadow of the opaque body produced by the candle, 

 which is at a distance from it of not more than 20 cm or 

 30 cm , may be much darker than the shadow produced 

 by the lamp, which is at a much greater distance. If 

 now the lamp be moved nearer to the opaque rod in a 

 straight line so that the shadow keeps the same position, 

 the intensity of the shadow can thereby not be altered, 

 because the shadowed portion continues to receive the 

 light of the candle which remains in its place ; but the 

 lamp throws now more light upon the illuminated part 

 of the screen than before, and by contrast the shadow 

 appears to become darker. 



The lines of the solar spectrum and the dark spectral 



