Dark specks that appear to float and move in front of the eye
It is quite common to see small specks, known as floaters, that appear to float in the field of vision. Although floaters seem to lie in front of the eyes, they are in fact fragments of tissue in the jelly-like vitreous humour that fills the back of the eye. These fragments cast shadows on the light-sensitive retina at the back of the eye. They move rapidly with any eye movement, but, when the eyes are still, they drift slowly.
The reason for most floaters is not known. They rarely affect vision, but you should consult your doctor immediately if floaters suddenly appear in large numbers or interfere with vision. A sudden increase in the number of floaters may indicate a serious eye disorder that requires urgent treatment, such as separation of the retina from its underlying tissue (see Retinal detachment), or a leakage of blood into the vitreous humour (see Vitreous haemorrhage).
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.