Wave–particle duality
Taylor's idea was to take a photo of the light coming out of the holes with a special camera that was unusually sensitive to light.  When bright light was shone through the holes, the photo showed an interference pattern, just like Young showed earlier.  Taylor then turned down the light to a very dim level.  When the light was dim enough, Taylor's photos showed tiny pinpoints of light scattering out of the holes.  This seemed to show that light was actually a particle.  If Taylor allowed the dim light to shine through the holes for long enough, the dots eventually filled up the photo to make an interference pattern again.  This demonstrated that light was somehow both a wave and a particle.