236 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
formerly used for the sea-salt ; this gives a very bright uniform yellow 
flame. For white light the improvements include two and threefold 
flat burners on the cylinder. Electric and incandescent lamps have also 
been used with good effect. 
To avoid optical activity in the liquid, the observation tube is 
enlarged in its centre and filled so as to permit the enclosure of an air- 
bubble, which, when the tube is in situ, does not interfere with observa- 
ion (fig. 36). 
