Crova's Disks
    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 250 francs

   This is Koenig's disk to illustrate ether (transverse) waves. The label is signed by Koenig. Notice that his initials resemble the "RK" symbol stamped into his turning forks. 

   The apparatus is at Union College in Schenectady, New York.

   The disks below are at Wesleyan University in Middletown, Connecticut; one of them illustrates transverse vibrations.

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