C. GENERAL PHYSICS. 191 



gree Fahrenheit." American Journal of Science, November, 

 1874. 



ON AN OPTICAL METHOD OF STUDYING THE VIBRATIONS OF 



SOLID BODIES. 



Professor O. N. Rood has devised a very simple and pre- 

 cise method of ascertaining the number of vibrations of solid 

 bodies, such as cords, rods, plates, bells, and membranes. He 

 takes a fork whose number of vibrations per second is accu- 

 rately known, and attaches to one of its prongs a short piece 

 of very fine wire. Another piece of wire is attached to the 

 body whose number of vibrations are to be compared with 

 the fork, and these two wires are brought near each other 

 with their lengths crossed. When the fork and the body 

 are now vibrated, the wires will vibrate in planes at right 

 angles to each other, and the intersections of the vibrating 

 wires, when viewed with a small telescope against a bright 

 background, will give an optical figure ; for, from the per- 

 sistence of impressions on the retina, the wires give a shaded 

 figure where, in their vibrations, they overlap. Thus each 

 musical interval, formed in the number of vibrations of the 

 two bodies, will give its own distinctive figure. From the 

 foregoing it evidently is easy with this method to bring a 

 vibrating string into unison with a given tuning-fork, or to 

 adjust it so that the interval shall be a quint, octave, twelfth, 

 or double octave, above or below. It is also easy to ascer- 

 tain the number of vibrations made by a string in a given 

 case, by the aid of a bridge and a properly selected fork 

 making: a known number of vibrations, the string: being; short- 

 ened by the bridge until it furnishes one of the above-men- 

 tioned figures, and hence executes a known number of vibra- 

 tions ; after which the number of vibrations made by its 

 whole length can readily be calculated from the law that 

 the numbers of vibrations of similar strings are inversely as 

 their lengths. 



After a string has, as above, been brought to make a known 

 number of vibrations per second, it may be used instead of 

 the standard fork; and by placing this string athwart a wire 

 attached to any other solid body, we can alter the length of 

 the string until they form unison, or a known interval with 

 each other; and then, on measuring the length of the string 



