3io OPTICAL PROJECTION 



potassium chlorate, 1 part shellac. For calcium : 2 parts 

 chalk, 10 parts potassium chlorate, 3 parts shellac. For 

 strontium : 3 parts strontium nitrate, 1 part potassic chlorate, 

 1 part shellac. For barium : 3 parts barium nitrate, 1 part 

 potassium chlorate, 1 part shellac. The shellac to be powdered 

 separately from the salts, which are also to be rubbed down 

 to powder, and the two mixed previous to use with a horn or 

 wooden spoon. 



These methods are superior in effect to the use of glass 

 rods, or sticks soaked in solution ; besides which- the manipu- 

 lation of such sticks in a jet is a very ticklish operation. 



179. Reversed Spectrum Lines, The absorption by vapours 

 of the same rays which they emit, is usually shown by sodium. 



FKJ. 170. Reversal of Sodium Line 



As a dark line depends upon the absorbing vapour being less 

 bright than the light of similar wave-length emitted by the 

 radiant, this experiment, of all others, is shown with marked 

 superiority by the electric arc. 



Fig. 170 shows the most widely known arrangement, as 

 adopted by Dr. Tyndall, except that two prism-bottles would 



