. The London, Edinburgh and Dublin philosophical magazine and journal of science. ;and this tends to produce the idea of constancy in the three elements, however, the first is found most impor-tant ; and the combined effect is that the image appears 18 or20 centim. distant, the judgment being quite uncertain. Theapparent area and depth of the external image are diminishedapproximately in the ratio of the diminished distance. Let the arms of the stereoscope be now pushed back tilla =—5°. Relaxation of the internal rectus and contractionof the external rectus muscles is habitually as
. The London, Edinburgh and Dublin philosophical magazine and journal of science. ;and this tends to produce the idea of constancy in the three elements, however, the first is found most impor-tant ; and the combined effect is that the image appears 18 or20 centim. distant, the judgment being quite uncertain. Theapparent area and depth of the external image are diminishedapproximately in the ratio of the diminished distance. Let the arms of the stereoscope be now pushed back tilla =—5°. Relaxation of the internal rectus and contractionof the external rectus muscles is habitually associated withrecession of the point of sight; and in this case it is carriedbeyond the limit of axial parallelism. But, again, the focaladjustment is for a point only 50 centim. in front; and thevisual angle is constant. Of the three elements the last isnow most important; and the combined effect is that the imageappears about 60 or 70 centim. distant, with correspondingincrease in apparent area and depth. In the accompanying figure (fig. 1) T have shown the result Fiff. of experiments conducted at different times during the lastfew months. The stereoscope was manipulated by an assistant,who varied the optic angle in the most irregular order, record-ing its values and my corresponding estimates of distance,while I was kept ignorant of these records until the entire listof estimates had been completed. Six independent series, ofeighteen estimates each, were made; and the curve A A ex-presses the mean result, the values of the optic angle beingtaken as abscissas and estimated distances as ordinates, the 442 On the Theory of Binocular Perspective. true distance of the stereograph being 50 centim. D D/ is thecurve whose equation is D = J a cosec J a, the value of a beingmy interocular distance, 60 millim. As might be expected,the probable error is large, being about + 8 millim.; and thecurve A A! is by no means regular, though its general importis unmistakable. It cuts the t
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Keywords: ., bookcentury1800, bookdecade1840, bookidlondon, booksubjectscience
