File:Carpenter's principles of human physiology (1881) (14595000269).jpg

Identifier: carpentersprinci00carp (find matches)
Title: Carpenter's principles of human physiology
Year: 1881 (1880s)
Authors: Carpenter, William Benjamin, 1813-1885 Meneses, Henry Power
Subjects: Human physiology Physiology
Publisher: London : J. & A. Churchill
Contributing Library: Francis A. Countway Library of Medicine
Digitizing Sponsor: Open Knowledge Commons and Harvard Medical School

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ur estimate of the Size ofnear objects, however, depends upon a more direct process, seems to be anecessary inference from the following very ingenious experiments, made byProfessor Wheatstone with a modification of his Mirror-Stereoscope, devisedfor separately testing the influence of the two conditions—namely, the magni-tude of the retinal picture and the degree of convergence of the optic axes—which are ordinarily in action together. When an object is moved nearerto or farther from the eye, its perceived or estimated magnitude undergoes nochange. But if two pictures, placed in the mirror-stereoscope, be made tomove to and from the mirrors, in such a manner as to vary their distancesfrom these (and therefore from the eyes), without altering the angle of con-vergence, their perceived magnitudes are augmented and reduced, in preciseproportion to the increased and diminished sizes of the retinal pictures.Conversely, if the two pictures be made so to change their places in regard to
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746 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. the mirrors (by moving in a horizontal circle, of which the middle-pointbetween the mirrois is the centre), that the angle of convergence is increased ordiminished, as it would be if the object were brought nearer to the eyes orremoved farther from them, the perceived magnitude of the pictures is alteredin an inverse manner; being reduced when the angle of convergence is in-creased, and increased when the inclination of the optic axes is lessened so asto approach parallelism. Thus it appears that the absence of alteration inthe perceived magnitude of an object as ordinarily seen at varying distances,is the result of the inverse action of these two kinds of suggestion; for theenlargement of the retinal picture when acting alone, occasions an increasein the perceived magnitude, whilst an increase of convergence, taking placeby itself, diminishes the perceived magnitude; and thus as these alterationsoccur simultaneously when an object

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