| Koenig produced "Crova's apparatus for representing
vibratory movements on the screen, with 8 disks". This was catalogue number
262 in his 1889 catalogue, and cost 400 francs ($80). The rotating disks
were held in a footed stand, and were turned, by a crank, past a vertical
slit. A beam of light, imaged by the optical system, impinged on the intersection
of the slit and the transparent portion of the slit, and as the disk
rotated, a spot of light showed the various oscillatory motions.
The system could demonstrate the propagation of a travelling
wave, the reflection of travelling sound waves, the reflection of the movement
of a continuous oscillation, the reflection of a standing wave, both the
fundamental and second harmonic, ether (transverse) waves, and the interference
of two vibratory motions.
A simpler model, with 7 disks and without the lenses, cost