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Phenomena visible in the entoptical field of the eye
This picture taken from Helmholtz' Treatise on Physiological Optics shows phenomena visible in the entoptical field due to the moisture secreted by the tear-glands. Entoptic phenomena are visual effects whose sources lie within the eye. This physical basis of the entoptic image is the main difference to optical illusions, which are merely based on cognitive interpretations of perceptual effects.
Helmholtz conducted his research using the self-invented ophthalmoscope, an instrument which made it possible to examine the inside of the human eye and was a major contribution to 19th century physiology.