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Picture of the Month (December 2009)
Dedicated User Captions:
1. c-beams glittering in the dark near the Tanhauser gate
2. The gorgeous sky over holiday planet Risa (by William T. Riker)
3. Starry night (by van Gogh)

Read more captions (not voted, latest on top)

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.

Related Items:
More about entoptical perception can be found here:
Czermak, Johann N. 1879. Ueber die entoptische Wahrnehmung der Stäbchen- und Zapfenschicht

For a catalogue of instruments used in ophthalmological research, have a look at:
Choquart & Peuchot. n. d. Optique Médicale et Scientifique: Instruments d'Ophtalmologie & de Laryngoscopie. Paris (Collection Paolo Brenni)

Picture Source:
Helmholtz, Hermann von. 1867. Atlas von elf Tafeln zu H. Helmholtz Handbuch der physiologischen Optik. Allgemeine Enzyklopädie der Physik IX. Band. Leipzig: Voss

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