and seeing that it was a soft October night,

Still from "La jetée"

The Shepard-Risset Glissando

A Shepard tone, named after Roger Shepard, is a sound consisting of a superposition of sine waves separated by octaves. When played with the base pitch of the tone moving upward or downward, it is referred to as the Shepard scale. This creates the auditory illusion of a tone that continually ascends or descends in pitch, yet which ultimately seems to get no higher or lower. It has been described as a “sonic barber’s pole”. 

Jean-Claude Risset subsequently created a version of the scale where the tones glide continuously, and it is appropriately called the continuous Risset scale or Shepard–Risset glissando. When done correctly, the tone appears to rise (or descend) continuously in pitch, yet return to its starting note. Basically, it’s a continuously descending tone that never gets any lower. It’s the acoustical version of M.C. Escher’s Penrose Stairs optical illusion. Source.