Disks to the Optical Analysis of Vibrating Bodies. 19 



experiments with the colour-circle and subjective colours; it 

 has no regulator. By winding up a strong spring by means of 

 the key a and the catch b, a blackened paper disk, B, of 5 to 8 

 inches diameter, with different series of concentric holes, is put 

 in rapid rotation. The velocity on starting has a maximum value, 

 and rapidly diminishes thence to zero. For obtaining great ve- 

 locity, disks of opake but not too stout paper should be taken, 

 such as was used by Magnus for observing the issuing jet ; for 

 by the action of the centrifugal force they set exactly in the plane 

 of rotation, and thereby find least resistance in the air; while 

 thicker disks, or metal disks, be they ever so thin, always cast. 

 The apparatus should be clamped on a table. 



Viewing a vibrating body, a singing flame, for instance, 

 through one of the series of holes, the above-named phenomena 

 are observed, though but transiently. In front of the disk is a 

 short piece of tube C on a stand Cs, which latter can be turned 

 about the joint s. The mouth of the tube can thus be placed in 

 front of any series of holes. By the vulcanized tube g, the disk 

 can be sounded by a bellows regulated by the foot, so as to obtain 

 the desired velocity by a coincidence in the sounds. 



It is true that some practice in continually adjusting the key 

 a is required in order to keep the velocity constant, in such a 

 manner that the apparent vibrations can be observed for minutes 

 together. A good deal of patience is required, especially in the 

 case of high notes. The rougher tuning is effected by the ear : 

 when unison is attained, the finer accord is effected, until the ap- 

 parent duration of a vibration is brought within from2 to5 seconds. 



But with an assistant the adjustment can be successfully ob- 

 tained in the following manner. The relation of the velocity 

 of the axis of the key a to the axis of the disk B is discovered, 

 once for all, by counting the teeth of the wheelwork. Suppose 

 now the key turned backwards to the stroke of a pendulum 

 — for instance, at the rate of one tooth of the check-wheel ; the 

 diskB will gradually acquire a constant velocity, at which just as 

 much runs off in the unit of time as is wound up by each check- 

 tooth. Now the interval of time for the winding up of each 

 check-tooth may be easily calculated from the height of tone of 

 the object investigated, the number of holes in the disk, and the 

 rotation of the teeth of the wheels. This interval of time may 

 be marked by a metronome, or a thread pendulum, which the 

 assistant can read. At each stroke of the pendulum, he has 

 to turn the key a tooth further, which is by no means difficult. 

 If the wheelwork is in good condition, by the increasing resist- 

 ance of the air and the inertia of the disk B, a condition of equili- 

 brium in the velocity of the disk is established which is adequate 

 for all experiments with tones which are not too high. At the 



C2 



