ACOUSTICS. 79 



A somewhat similar arrangement is made for the 

 other side, save that the friction pulley / has its bear- 

 ing made fast in a separate piece c, which is so fastened 

 to the end of a long screw d that the whole fixture can 

 be moved to or from the centre of the plate a. The 

 piece c is furnished with two guides, which keep it 

 steady in any place where it is put. The mirror m' is 

 made to tilt in a perpendicular plane by an arrange- 

 ment quite similar to the former one, save that the wire 

 connection has its lower end bent into a horizontal 

 loop, through which a pin in the face of the pulley / is 

 thrust. This is practically an eccentric, and, being 

 directly fastened to the hinge of the mirror m r , gives to 

 it an angular motion proportional to the distance of the 

 pulley face-pin from the centre. The mirrors should 

 be not less than two inches square. If then the pin is 

 an eight of an inch from the centre of the friction pul- 

 leys, they will have ample angular motion j much larger 

 than can ever be got from forks. 



Experiments. It is evident that if the two friction 

 pulleys have equal diameters, and they are at equal dis- 

 tances from the centre of the plate a y they will vibrate 

 in unison in their respective planes. Now let a beam 

 of light r, from the porte lumiere, fall upon the mirror 

 m at such an angle as to be reflected first upon the 

 mirror m', thence to the screen. If the plate a is now 

 revolved, the beam of light will describe a circle, an 

 ellipse or a straight line, either of which can be made 

 at will by simply adjusting the crank of one of the 

 mirrors to the required angle. Thus, suppose the 

 mirror m r is tipped back its farthest by bringing the 

 pulley pin at the top, as indicated in the drawing, at 

 the same time that the mirror m is at its maximum an 



