The Philosophical magazine; a journal of theoretical, experimental and applied physics . nt retaining its position. Hence it follows^ thaton the part of the retina in the axis of vision, all rays, howeverobliquely incident, give the same visible direction perpendi-cular to the surface of the membrane. That the same pro-perty is possessed by every other part of the retina cannot bedoubted, and may be proved by direct experiment. Although DAlembert states it as unquestionable, that whenthe visual ray is in the axis of vision, or the optic axis, andpasses to the retina without refraction, the poi
The Philosophical magazine; a journal of theoretical, experimental and applied physics . nt retaining its position. Hence it follows^ thaton the part of the retina in the axis of vision, all rays, howeverobliquely incident, give the same visible direction perpendi-cular to the surface of the membrane. That the same pro-perty is possessed by every other part of the retina cannot bedoubted, and may be proved by direct experiment. Although DAlembert states it as unquestionable, that whenthe visual ray is in the axis of vision, or the optic axis, andpasses to the retina without refraction, the point which emitsit will be seen in the direction of a line passing from its imageto the visible point; yet, after he has found that his mecha-nical principle is not correct, he gives loose reins to his scep-ticism, and maintains the extraordinary paradox, that objectseven which are placed in the optical axis are not always seenin this axis. The following is the argument he employs, whichI shall give in his own words. If we direct the two optic axes A E, B E, fig. 2, towards Fig. 2. -A-. a star E, it is certain that this star appears much nearer to usthan it really is: it is true that we estimate its distance onlyin a very imperfect and vague manner; but it is not less cer-tain that this distance perceived, whether apparent or pre-sumed, is greatly helo-w the real distance. If, then, we see thestar in each of the optical axes A E, B E, we should see it ineach of these axes in the points e, e, which are incomparablynearer A and B than E. Thus we should see two stars e^ e,and their apparent distance e e would be nearly equal to A , however, proves that we see only one star, andthis star is seen nearly at the middle point s of the line e e the direction of lines A e, B s, different from the optic is true that these lines, though really different from the opticaxes, deviate from them but very little, but still they do differfrom them; and this experiment is suffic
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