128 O. N. Rood—Optical method of studying the 
or below. It is also easy to ascertain 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 shortened till it furnishes one of the above 
mentioned figures, and executes hence a known number of 
vibrations, after which the number of vibrations made by its 
whole length can readily be calculated by a well known law. 
Vibrating cords.—To bring two cords into unison, or to pro- 
duce one of the above mentioned intervals, it is not at all nec- 
known for his excellent workmanship, merely adding a —— 
to one of the bridges for the purpose of holding down the co 
But this instrument, although admirable when the ear is used + 
some modification of the older arrangements of Weber or 
Fischer would be found to answer better. 
Vibrations of rods, bars and plates.—It is evident that rods or 
bars supported at one extremity or at two nodes, and provided 
with fine terminal wires, can by this method be brought into 
unison, or have one of the above mentioned intervals estab- 
RE ek ON sr RE POP Ae an aE ee 
‘ee 
