60 THE ART OF PROJECTING. 



lens, as represented in Fig. 38. First, with the fork 

 held as indicated, make it to vibrate. The string will 

 divide up into segments, all of which can be plainly seen 

 and counted. Second, turn the fork so that it vibrates 



Fig. 38. 



in a horizontal plain. The number of segments will 

 be doubled. Third, hang another button upon the pin- 

 hook, so that the weight will be doubled. Count the 

 segments while the fork vibrates, both perpendicularly 

 and horizontally. In this way some of the laws of 

 vibrating strings can be demonstrated. 



Fasten a small piece of wire to one prong of the tun- 

 ing-fork, and when the latter is vibrating draw it quickly 

 across a piece of smoked glass. The undulating line 

 will show well when projected. 



THE OPEIDOSCOPE. 



Take a tube, of any kind, that is five or six inches 

 long and an inch or more in diameter, tie a thin rub- 

 ber membrane or a piece of tissue-paper over one end, 

 and on the middle of the membrane glue a piece of 

 looking-glass that is not more than the eighth of an 

 inch square. The light from the porte lumiere falls 



