I.] 



PRINCIPLES AND METHODS. 



that the image thus built up will take the form of a line or circle, 

 according to the slit we use. 



Many chemical substances, salts of various metals, become lumin- 

 ous by inserting them into flames, as we have treated common salt 

 (chloride of sodium). With each metal the colour imparted to the 

 flame is different. The resulting spectrum is called a discontinuous 

 spectrum, because it is only here and there that images of the slit are 

 produced ; because some coloured rays, and not all, are present. 



FIG. 4. The spectrum of a complicated light-source as seen with a circular 

 and a line slit. 



The usual laboratory arrangement for^ observing the spectra of 

 flames, is shown in the woodcut (Fig. 5). 



Further, the system of images of the needle (or slit) varies for each 

 substance, and it is on this ground that the term spectrum analysis is 

 used, because we can in this way recognise the various substances in 

 the flame. 



Fia. 5. Observation of a flame spectrum -with ordinary spectroscope with com- 

 parison prism, a, prism ; b, collimator ; d, slit ; e e, flames to be compared ; 

 /", observing telescope ; g, scale illuminated by h and reflected by the second 

 surface of the prism into the telescope. 



