The nature of light has always been a subject of wonder and speculation. The idea that light consists of particles can be traced back to the Greeks-particles either coming from the visible object or projected from the eye to feel the object, though Aristotle held that light was action in a medium. Descartes thought it was a pressure transmitted through his plenum, and Hooke suggested that it was a rapid vibration in a medium, a theory worked out in some detail by Huygens.