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) has shown that when a subject fixates as steadily as possible upon a well-defined target, certain involuntary eye-rotations persist.The natural involuntary movements of the eye include (i) a high frequency tremor of amplitude less than 0 5 minute of arc (min . arc) and frequencies up to 150 c/s, (ii) intermittent rapid flicks of up to 50 min. arc occurring at irregular intervals (from 0 03 to 5 sec), and (iii) a slow motion drift at the rate of about 1 min . arc/sec in the interflick periods.It is convenient to use a common measure for rotations of the eye, size and motion of the target and for movements of the retinal image. This is obtained by referring them all to equivalent angles in the visual field. The minute of arc is taken as the unit angle, the second being used only as a unit of time. At the fovea, 1 min. arc corresponds to a distance of about 5,u; the distance between the centres of adjacent cones is about 06 min . arc.The fact that these involuntary movements exist does not prove that they have any important function in visual perception. However, by means of a device previously described (Ditchburn & Fender, 1955) it is possible to produce a visual target which moves so that its image remains on the same part of the retina despite involuntary eye movements. We call this a stabilized retinal image. When the subject views a stabilized image, perception of detail fails intermittently. For example a black bar a few min.arc wide on a circular field 60 min. arc in diameter (in the centre of the fovea) and 50 m-L brightness would be seen easily and continuously in normal vision. However, when the image is stabilized the bar appears to fade out leaving the whole field uniformly illuminated; the bar reappears after a short interval and then disappears again, the whole process of 'fade-out' followed by spontaneous regeneration being repeated at irregular intervals with a median time of about 5 sec for the subject used in the experiments reported below.

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