A text-book of physiology for medical students and physicians . stinctly a thing that has no real existence,—a case, therefore,in which we can not trust our senses. The delusion seems to bedue to the fact that the two lines, in the position indicated, forma projection on the retina such as would be made by an actualvertical rod placed at the point at which we see one. Fig. 163gives an interesting illustration of the way in which our judg-ment of solidity may vary with our interpretation of mathe-matical perspective and shading when these factors are arrangedto give more than one choice. If the
A text-book of physiology for medical students and physicians . stinctly a thing that has no real existence,—a case, therefore,in which we can not trust our senses. The delusion seems to bedue to the fact that the two lines, in the position indicated, forma projection on the retina such as would be made by an actualvertical rod placed at the point at which we see one. Fig. 163gives an interesting illustration of the way in which our judg-ment of solidity may vary with our interpretation of mathe-matical perspective and shading when these factors are arrangedto give more than one choice. If the figure is looked at steadilyit may assume several different appearances; two are especiallyprominent. We may see two cubes resting upon a third one,each with the black side undermost, or we may see one cube rest-ing on two under ones, each with its black side uppermost. Ourjudgment in the matter changes from one interpretation to theother without any apparent cause. That the act of accommoda-tion plays a part in the changes is shown by the fact that if one. Fig. 166.—Figure to illustrate binocular deceptions depending upon different inter-pretations of the mathematical perspective and the lights and shades. On gazing fixedlythe image will change from a single cube with black top resting on two others with blacktops, to one of two cubes with black bottoms resting upon a single cube with blackbottom. Still other figures may appear from time to time. focuses for the point a, this point may be held in the foregroundand the second of the above appearances be seen. While if theeyes are accommodated strongly for point b, it will be broughtforward and the first of the two appearances described is broughtinto view. PHYSIOLOGY OF THE EAR. CHAPTER XX. THE EAR AS AN ORGAN FOR SOUND SENSATIONS. In discussing the physiology of the ear it is necessary to considerthe functional importance of its various parts, the external earconsisting of the lobe or pinna, the external auditory meatus,
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