If the light is moved at about 1 Hz, adaptation is defeated, and a clear image can be seen indefinitely. Unless the light moves, the image disappears within a second or so. The unusual angle casts the image onto unadapted portions of the retina. Normally the image of the retinal blood vessels is invisible because of adaptation. It can be seen by shining a bright, moving light like a penlight onto the sclera (the white of the eye) in a darkened room.
Floaters or muscae volitantes are slowly drifting transparent blobs of varying size and shape, which are particularly noticeable when lying on the ground looking up at the sky.Some examples of entoptical effects include the following: During the 1920s, some theosophists, unaware of the physical explanation, maintained that the moving spots seen in the blue field entoptic phenomenon were "vitality globules" related to the concept of prana in yoga. Thus, entopical phenomena are experienced subjectively.
Helmholtz comments on phenomena which could be seen easily by some observers, but could not be seen at all by others. However, like optical illusions or hallucinations, the observer of an entoptical effect cannot give others a direct view of what he or she observes. Most entoptical phenomena have a direct, known physical cause.
They are different from optical illusions, which are perceptual effects occurring within the brain. (Occasionally, these are called entopic phenomena, which is probably a typographical mistake see entopic.) Under suitable conditions light falling on the eye may render visible certain objects within the eye itself. Entoptic phenomena are visual effects whose source is within the eye itself.
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