Microsacaddes (lit. trans. 'tiny pulls') are very small involuntary eye movements that are ubiquitous in mammalian vision. They've been known-about and studied since the mid 1880s.

When fixed on any particular scene, the eyes tend to rapidly and repeatedly 'scan' the scene over a tiny range of less than 1 degree of the visual field.

The purpose of these movements has been discussed since their discovery, but an overall agreed explanation is still lacking.

Some experiments appear to show that the microsacaddes can improve visual acuity - while other studies suggest they can actually blur vision.

Other theories suggest that they help to prevent a retinal image from fading due to saturation of the retinal cells. or that the constant neuronal firing keeps the visual 'circuits' active.

The accumulated evidence demonstrates that microsaccades serve both perceptual and oculomotor goals and although in some cases their contribution is neither necessary nor unique, microsaccades are a malleable tool conveniently employed by the visual system."

Source : Vision Research 49 (2009) 2415–2441- which provides a comprehensive roundup of the current theories.