492 EXPERIMENTS ON LIGHT-SPECTRA. 



the left end of the spectrum seen. The distance of the 

 prism from the flame should not be less than l m . By 

 means of fig. 279, it will be more easy to find the 

 direction in which the flame is seen ; the latter is 

 denoted by /, the prism by p, and p o is the direction 

 of the ray which reaches the eye. When the prism is 

 turned slightly to and fro, its refracting edge remaining 

 in the same position, the deviation will somewhat vary ; 

 this follows from what has been stated on v page 462 ; 

 it is best to give that position to the prism in which it 

 produces the least deviation. 



The image of the flame when the latter is coloured 

 by common salt appears laterally displaced, but in all 

 other respects it is like the original when viewed with- 

 out the prism : the hot vapour of the salt emits only 

 yellow light of a definite refrangibility which is the 

 same for all its rays ; hence all its rays are equally re- 

 fracted, and reproduce an undistorted image, just the 

 same as if no refraction whatever had taken place. Com- 

 mon salt consists of a gas, chlorine, and the metal sodium. 

 It is the vapour of sodium which colours the flame 

 yellow, and as this metal is contained in a great many 

 substances for example in common soda, in Glauber's 

 salts, in most kinds of glass, &c. all these substances 

 when heated in the hydrogen flame colour the flame 

 yellow. Of glass only the smallest trace is evaporated, 

 but this is quite sufficient to produce the characteristic 

 yellow colour. 



Another substance, lithium, emits in the state of 

 vapour light of a beautiful carmine colour, which also 

 has only one refrangibility. If a small quantity of some 

 compound of lithium for example, lithium carbonate 



