Vibration Microscope
  The vibration microscope is used to calibrate the frequency of a tuning fork in terms of a tuning fork of known frequency. The original vibration microscope was invented by Lissajous, and the form in which the reference tuning fork is driven electromagnetically is due to Helmholtz. 

   The apparatus at the right is at Amherst College in Massachusetts, and is listed at 140 francs in the 1889 Koenig catalogue (Cat. No. 234i).

   In his 1900 book, Sound and Music, the Rev. J. A. Zahm of Notre Dame noted that the device "is composed of an electric fork, attached to a solid support, and a microscope. The object of the microscope is borne by one of the prongs of the fork [in the apparatus above it is attached to the lower fork]... When the fork is set into motion the objects visible in the field of the microscope seem to move in the same direction as does the fork. If now a second tuning-fork, whose prongs are perpendicular to those of the first, be caused to oscillate, a point on the second fork will appears to describe a curve, whose form will depend on the vibration-frequencies of the two forks used. If the intervals of the forks be perfect, [simple Lissajous figures] will appear... If, however, the interval be perturbed in any way by a change in the temperature of the forks, for instance, the figure is no longer constant. It immediately begins to pass through a cycle of changes... The longer the time required for effecting a complete cycle of changes, the nearer the intervals of the forks are perfect. [Consider a vibration microscope] made to execute exactly 128 vibrations per second. If ...[the figure] goes through a cycle of changes in 10 minutes, it means that our comparator executes 10x60x1828 = 76800 vibrations, while the other fork, during the same period, makes one vibration more or less than this number." From J. A. Zahm, Sound and Music, second edition (A. C. McClurg & Co., Chicago, 1900) pp 418-419.
   This slightly different vibration microscope is at the Wesleyan University in Middletown, Connecticut. The eyepiece of the microscope is carried on a wooden block, and the objective lens on the tine of the tuning fork has been swung down slightly. 

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   This Koenig vibration microscope at Cornell University appears to be a variation on the two examples above. The optical system is mounted on the upper side of the tuning fork on the right-hand side.

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