Identifier: textbookofphysio1916howe (find matches)
Title: A text-book of physiology : for medical students and physicians
Year: 1916 (1910s)
Authors: Howell, William H. (William Henry), 1860-1945
Subjects: Physiology Medicine
Publisher: Philadelphia and London : W. B. Saunders Company
Contributing Library: Yale University, Cushing/Whitney Medical Library
Digitizing Sponsor: Open Knowledge Commons and Yale University, Cushing/Whitney Medical Library

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e images disappears. The reason for the cross-projec-tion of the images is made apparent by the construction in Fig. 159,/, bearing in mind the essential fact that in projecting our retinalimages we always project to the plane of the object upon which theeyes are focused. In the figure the eyes are converged on A; theimages of point B fall to opposite sides of the line of sight and areseen double and are projected to the plane of A, the image on theright eye being projected to bf on the left of A and that on the left eyeto b on the right of A. In a similar way it may be shown that ob-jects farther away from the eye than the point looked at are doubledhomonymously,—that is, the right-hand image belongs to the right-eye, and the left-hand one to the left eye. The fact is explained bythe construction in Fig. 159, II, in which A is the point convergedupon and B the more distant object. In all binocular vision, there-fore, the series of objects between the eye and the point looked at are
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Fig. 159.—Diagrams to show homonymous and heteronymous diplopia: In / the eyesare focused on A; the images of B fall on non-corresponding points,—that is, to differentsides of the fovese,—and are seen double, being projected to the plane of A, giving heter-onymous diplopia. In // the eyes are focused on the nearer point, A, and the farther point,B, forms images on non-corresponding points and is seen double,—homonymous diplopia,—the images being projected to the focal plane A. doubled heteronymously, and those extending beyond the point inthe same line are doubled homonymously. Normally we take noconscious notice of this fact, our attention being absorbed by theobject upon which the lines of sight are directed. Some physi-ologists, however, have assumed that the knowledge plays an im-portant part subconsciously in giving us an idea of depth or per-spective,—an immediate perception, as it were, of the distinction BINOCULAR VISION. 375 between foreground and background. It i

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