| The apparatus at the right is listed
as catalogue number 169 in the 1889 Koenig catalogue as a "string stretched
before the slit of a resonance box of variable volume... 26 francs."
In use, the string is bowed (probably on the right-hand side where it is accessible) and the particular frequency of sound is communicated to the box. I was not able to examine the box to see how its volume can be varied. The box probably acts as a Helmholtz Resonator.
This apparatus is in the collection of Amherst College in Massachusetts.