| 21. | Later Hermann von Helmholtz and Ewald Hering worked out the exact shape of the horopter almost at the same time.
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| 22. | These early empirical investigations used the criterion of singleness of vision, or absence of diplopia to determine the horopter.
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| 23. | The empirical horopter is flatter than predicted from geometry at short fixation distances and becomes convex for farther fixation distances.
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| 24. | Geometrically the horopter is a circle passing through the nodal point of the two eyes and through the fixation point.
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| 25. | This was observed by Hering and Hillebrand at the same time, as well as Helmholtz for the vertical horopter.
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| 26. | But this is true only for short fixation distances where the empirical horopter is intermediate between these two set of points.
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| 27. | At first glance, it seems that Aguillon discovered the geometrical horopter more than 200 years before Prevost and Vieth and Muller.
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| 28. | With little time to do research and even scarcer financial resources, he turned to binocular vision and the problem of the horopter.
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| 29. | The output signals from the LGN determine the spatial dimensions of the stereoscopic and monoscopic portions of the horopter of the visual system.
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| 30. | At some given distance, called the abathic distance, the empirical horopter becomes a straight line, thus matching the apparent fronto-parallel plane.
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