R C and L C, as the attention is successively directed to different points in the field of view. If accepted at all, it must be accepted fully. If we suppose the semi-lenses removed, and that R and L together represent a binocular camera, the diagram shows the exact Fig. 13.—Theory of Visual Triangulation. relation between this and an object to be pictured, and the admirable mathematical discussion which Helmholtz gives subsequently in full is strictly applicable. But, if the observer's eyes be too near together, or the stereographic interval be too great, the relation between the visual lines ceases to be the same as that between the camera axes, and we no longer have the conditions under which the geometric discussion can be applied. It is but due to Helmholtz to add that he closes with the following remark: "These conditions are not generally fulfilled for the photographic proofs and the stereoscopes of commerce." The same credit can not be given to the writers of the ordinary text-books. This qualification is of the last importance, for without it the theory is absurd, the apparent position of the image determined by intersection of visual lines being behind the observers head when optic divergence is induced, and at an infinite distance when they are parallel. But, even when camera axes and visual lines bear the same relation among themselves, the abnormal muscular condition necessitated in stereoscopic vision introduces a disturbing element. The theory is hence not applicable at all to the stereoscope, but must be limited to the discussion of the binocular camera.

With a view to enabling persons with untrained eyes easily to perform many of the experiments through which variation in appearance of the binocular image is produced by varying the conditions under which the same stereograph is viewed, the writer has devised an adjustable stereoscope (Fig. 14), which presents the additional very important advantage of rendering vision as nearly painless as it can be with the ideal stereograph, even although the stereographic interval on the one employed be so great as to produce only confusion, or strain of the eyes, when the common form of stereoscope is used. Instead of being fixed in position, the semi-lenses are lightly rested in a pair of boxes, with openings in front and rear so as to transmit the light. Attached to the partition between them are a pair of springs against which the thin edges of the semi-lenses are pressed by adjusting-screws in contact with their thick bases. By turning these so that the glasses are pressed as close as possible together, the light which enters the eyes passes through the thicker part of each glass, where the planes that may be supposed to touch the opposite curved