234 



PROCEEDINGS OF THE AMERICAN ACADEMY 



slotted axle A B and the slotted shaft connected with E until they are 

 in position. The string is tightened hy means of pulley at p, Fig. 1. 



No. 2. — Method by Beam of Light. 



Two wires are stretched at right angles to each other, and are 



* maintained in vibration by electro-magnets, A and B, placed at the 



middle of their half-lengths, and by an elastic piece of wire, w and », 



placed at their middle points. The stretched wires are provided with 



Fig. 3. 



mirrors at c and d, which are steadied by flexible brass strips attached 

 to the supports upon which the wires are stretched. The horizontal 

 wire is kept in vibration by one battery, and the vertical by another. 

 The process of tuning the wires can be illustrated to an audience by 

 increasing the tension of the horizontal wire by means of a key at K. 

 When the wires are tuned, a symmetrical and steady figure is pro- 

 duced on the screen, of very large size. The lime light is so placed as 

 to allow the beam of light to fall on the mirror d, and then on the 

 mirror c. 



