ni.] CONTINUOUS AND DISCONTINUOUS SPECTRA. 31 



nicely arranged the appearance of the spectrum will be so 

 entirely changed that a beginner will be apt to fancy that 

 something has gone wrong. Nothing has gone wrong however, 

 we have simply passed from the spectrum of polychromatic 

 to that of monochromatic light from white light to coloured 

 light ; instead of 



\\7 il i^> (T* ^7 fr^\ r ^> 

 W U LE; (^\ \( (&) LT.< 



we get light in the yellow at 



' " 



Next try the circular slit. Instead of a line we get a circle in 

 the same part of the spectrum. 



What we shall see in passing from the spectrum of the candle 

 to the spectrum of the sodium vapour as seen by a straight 

 or a circular slit is shown in the accompanying woodcut. 



I I s I I I 3 ' ' 

 :- S -I ,1 ; IS '!:..* 



FIG. 14. The spectra of continuous and discontinuous light-sources, the latter 

 seen with a line and circular slit. 



If we treat a salt of lithium in the same manner we shall see 

 with the line slit a bright line in the red, with the circular slit 

 we shall get a circle in the same place. A salt of thallium will, 

 according to the slit we use, give a bright line or circle in the 

 green, and if we examine a very complicated light source we 

 shall arrive at the same result a spectrum characterised by a 

 large number of bright lines or circles, depending upon the shape 



