in.] A LINE SLIT IS BEST. 33 



if we use, say, a circular slit but when we come to substances 

 that give very complex spectra, or when we use only a low dis- 

 persion, it is evident we shall get the result shown in the last 

 figure. The different coloured circles will overlap each other 

 and the whole spectrum will be in a state of confusion. For 

 the sake of comparison the same spectrum is shown as observed 

 with a line slit. When a line slit is used we can make it as 

 narrow as we please, and so reduce the overlapping to a minimum 

 and get a spectrum of the greatest purity. 



After these statements the meaning of the term " line spectra " 

 should not present any difficulty. 



We may now then dismiss this part of our subject, in which 

 Wollaston was the pioneer, with the remark that spectra of 

 bright lines are produced by the giving out of light by light- 

 sources which are coloured. In other words, the radiation of 

 coloured light-sources when examined by the spectroscope gives 

 us bright line spectra. 



