Floating Number

This exhibit is a box into which the visitor peeps after pushing a button. Following the button push, a strobe of light is flashed into the eye of the visitor. This flash creates an afterimage of a coloured number that lasts for several minutes. Wherever the visitor turns his or her gaze, the afterimage follows.

After being dazzled by a very bright light, the photoreceptors of the eye continue to respond for a short while after the flash ends, so we see a bright after-image. Following this, the response of the receptors is suppressed and we see a darkened after-image that changes colours as it fades, since the receptors for different colours recover at different rates.

After being strongly stimulated, the eye's photoreceptors (and perhaps other cells in the retina or even in the cortex) continue to respond for a short time. This results in a positive afterimage of the same colour as that of the flash. The positive afterimage fades quite rapidly and is replaced with a negative aftereffect. This aftereffect is the suppression of the sensitivity of one or more of the levels noted, or adaptation, resulting from the strong preceding response. It manifests itself in an image that is the colour- complement of the stimulus when a white wall is looked at. This is because those colour-channels stimulated by the flash now respond less to the white wall than the others, so the image appears to have the colour of those that continue to respond, namely, the complementary colour to that of the flash. The image fades slowly as the suppressed colour channels regain sensitivity.