Many visual problems require a better understanding of where people look in a given situation. Most of these questions can be answered by using a simple head-mounted optical device. This consists of a small movie camera and a periscope that transmits a spot of light reflected from the eye. As the subject moves his head and eyes, the equipment photographs the changing scene. In the camera the image of the scene is instantaneously marked with a spot that indicates the position of the line of sight. The effects of eye movements can be distinguished from those of head movements. (1) Immediate recordings can therefore be taken; (2) the subject can turn his head; (3) the scene can be rapidly changing all the time.
NORMAN H. MACKWORTH and EDWARD LLEWELLYN THOMAS, "Head-Mounted Eye-Marker Camera," J. Opt. Soc. Am. 52, 713-716 (1962)