THE SPECTRUM. 199 



hofer's lines, and they excited great interest, though the 

 cause and the significance of them remained for a long time 

 an unfathomable mystery. 



Fraunhofer gave names to the principal lines that he ob- 

 served as soon as he found that they were con- 

 stant and unchangeable in the solar spectrum, 

 designating each by a letter of the alphabet. 

 The names and positions of some of the princi- 

 pal ones those that were first discovered are 

 1 1 given in the engraving. 



The white band on the black ground repre- 

 sents the spectrum itself, and the lettered cross- 

 bars Fraunhofer's lines. It is only a very few of 

 the principal ones that are thus named. Fraun- 

 hofer himself discovered and mapped many hun- 

 dred of them, and the number has since been ex- 

 tended to several thousand. 



It is now found that these lines depend upon the nature 

 and the chemical composition of the incandescent sub- 

 stance from which the light proceeds. They are always 

 the same for the light which comes from the sun, but dif- 

 ferent for the different kinds of artificial light ; always the 

 same, however, for the same kind of light. There are cer- 

 tain lines, it is found, that are characteristic of certain sub- 

 stances, and when these substances exist in any flame, or 

 other incandescent source of light, the lines pertaining to 

 them ai-e sure to appear. Thus these lines in any spec- 

 trum constitute a language by which those who have 

 learned it can determine with great certainty what sub- 

 stances exist in any flame the spectrum of which they have 

 the opportunity to examine. 



To examine the spectra of flames in this way requires a 

 somewhat complicated and delicate, and very exactly made 

 instrument. This instrument is called the spectroscope. 



