98 THE ART OF PROJECTING. 



has been stirred to give a turbidity to it, as the beam 

 will be traced much better ; a few drops of milk or 

 of chalk-dust will answer the same purpose. A little 

 smoke in the air will serve to mark the course of the 



Fig. 73. 



beam till it reaches the water, where its direction will 

 be seen to change, becoming more perpendicular. Re- 

 flect the beam upon the water at various angles of 

 incidence and mark the course of the refracted rays. 



In place of the small tank take a beaker, or any 

 vessel with square glass sides, and reflect the beam 

 upon the surface of the water as before. 



The experiment may be varied by filling the vessel 

 half full of water, then carefully pouring strong alcohol 

 upon it to the depth of an inch or two, being careful 

 not to mix them while pouring, and some ether upon 

 the alcohol in the same careful way. Sending now the 

 beam through them as before, the different refractive 

 powers of the various liquids will be seen. 



3 HEATED AIR. 



Fig. 74. 



