Optical writers who lived after the time of Aguilonius seem to have considered the subject of binocular vision as exhausted in his admirable work. Gassendi,[14] though he treats the subject very slightly, and without any figures, tells us that we see the left side of the nose with the left eye, and the right side of it with the right eye,—two pictures sufficiently dissimilar. Andrew Tacquet,[15] though he quotes Aguilonius and Gassendi on the subject of seeing distances with both eyes, says nothing on the binocular vision of solids; and Smith, Harris, and Porterfield, only touch upon the subject incidentally. In commenting on the passage which we have already quoted from Leonardo da Vinci, Dr. Smith says, “Hence we have one help to distinguish the place of a near object more accurately with both eyes than with one, inasmuch as we see it more detached from other objects beyond it, _and more of its own surface, especially if it be roundish_.”[16] If any farther evidence were required that Dr. Smith was acquainted with the dissimilarity of the images of a solid seen by each eye, it will be found in his experiment with a “long ruler placed between the eyebrows, and extended directly forward with its flat sides, respecting the right hand and the left.” “By directing the eyes to a remote object,” he adds, “the _right side of the ruler seen by the right eye_ will appear on the left hand, and _the left side on the right hand_, as represented in the figure.”[17] [14] _Opera_, tom. ii. p. 394. Lugduni, 1658. [15] _Opera Mathematica Optica_, tribus libris exposita, p. 136. [16] _Opticks_, vol. ii., Remarks, pp. 41 and 245. [17] Id., vol. i. p. 48, Fig. 196. In his Treatise on Optics, published in 1775, Mr. Harris, when speaking of the visible or apparent figures of objects, observes, that “we have other helps for distinguishing _prominences of small parts_ besides those by which we distinguish distances in general, as their degrees of light and shade, and _the prospect we have round them_.” _And by the parallax, on account of the distance betwixt our eyes, we can distinguish besides the front part of the two sides of a near object not thicker than the said distance, and this gives a visible relievo to such objects, which helps greatly to raise or detach them from the plane in which they lie. Thus the nose on a face is the more remarkably raised by our seeing both sides of it at once._“[18] _That is, the relievo is produced by the combination of the two dissimilar pictures given by each eye._