90 



wires in the telescope give a fixed point, which can be brought upon 

 any line in the spectrum so as to register its position. 



In this instrument you will observe that there are two prisms side by 

 side. The ray of light passes through both and is thus spread out into 

 a much broader band ; the number of prisms is sometimes increased to 

 3uch an extent that the emergent ray crosses its original path. The 

 prisms may also be so arranged that the ray of light passes out of the 

 telescope in a direct line from its source. Such an instrument is termed 

 a "direct vision" spectroscope and is very handy for general work, but 

 is not suitable for very delicate research. 



It had long been known that certain substances when burnt produced 

 variously coloured flames. Thus I have here some of the red fire of the 

 pyrotechnist and you will see what a splended crimson flame it produces. 

 Fox Talbot examined this flame (among others) with the prism, and dis- 

 covered that its spectrum was crossed by brilliantly coloured lines. 



Soda salts produce a yellow line and Swann very nearly discovered 

 the true cause, but the absolute discovery remained for Bunsen and 

 Kirchoif. They found that all sodium salts gave this yellow line, and 

 that no other substance that was perfectly free from sodium did give the 

 same line ; hence they rightly argued that the presence of this line in the 

 spectrum was indicative of the presence of sodium in the flame. They 

 were not long in generalizing upon this fact, and found that every 

 elementary substance which could give rise to a flame in burning, produced 

 a definite set of lines which always occured of the same colour, in the same 

 position, and of the same strength ; and they termed these lines the 

 spectrum of the element. They carefully mapped all the lines produced 

 by the then known elements and used these maps to discover the nature 

 of unknown substances, just as the mariner uses his map to discover his 

 whereabouts in approaching an unknown shore. Their work soon bore 

 remarkable fruit, for in examining the residue from a German mineral 

 water they saw some lines which were new to them, and immediately 

 concluded that they had dropped across an unknown element. And so it 

 turned out ; for after patiently evaporating 44 tons of the Durkheim 

 spring, they got 200 grains of salts, of what they found to be two new 



