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the section of the sphere as seen by both eyes.1 These three pictures of the solids are all dissimilar. The right eye E does not see the part Blcif of the sphere ; the left eye does not see the part Blcga, while the part seen with both eyes is the hemisphere Blcgf, the dissimilar segments Bfg, Cgf being united in its vision.2

After demonstrating his theorems on the vision of spheres with one and both eyes,3 Aguilonius informs us, before he proceeds to the vision of cylinders, that it is agreed upon that it is not merely true with the sphere, but also with the cylinder, the cone, and all bodies whatever, that the part which is seen is comprehended by tangent rays, such as Eb, Ec for the right eye, in Fig. 3. "For," says he, "since these tangent lines are the outermost of all those which can be drawn to the proposed body from the same point, namely, that in which the eye is understood to be placed, it clearly follows that the part of the body which is seen must be contained by the rays touching it on all sides. For in this part no point can be found from which a right line cannot be drawn to the eye, by which the correct visible form is brought out."4

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,8 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

1 It is obvious that a complete hemisphere is not seen with both eyes.

'Aguilonius, Oplicorum, lib. iv. pp. 306, 307.

3 In the last of these theorems Aguilonius describes and explains, we believe for the first time, the conversion of relief in the vision of convex and concave surfaces. See Prop. xciv. p. 312.

* Id., Id., p. 313.

5 Opera, tom. ii. p. 394. Lugduni, 1658.

f eye, and the right side of it with the right eye,—two pictures sufficiently dissimilar. Andrew Tacquet,1 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."2 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."3

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 tlvan the said distance, and thu 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."1 That is, the relievo u produced by the combination of the two dissimilar pictures given by each eye.

1 Opera Maihematica Optica, tribus libria exposita, p. 136.

8 Optlckt, vol. ii., Bemarks, pp. 41 and 24S. 'Id., ml 1. p. 43, Fig. 196.

Without referring to a figure given by Dr. Porterfield, in which he actually gives drawings of an object as seen by each eye in binocular vision,2 the one exhibiting the object as seen endwise by the right eye, and the other the same object as seen laterally by the left eye, we may appeal to the experience of every optical, or even of every ordinary observer, in support of the fact, that the dissimilarity of the pictures in each eye, by which we see solid objects, is known to those who have never read it in Galen, Porta, or Aguilonius. Who has not observed the fact mentioned by Gassendi and Harris, that their left eye sees only the left side of their nose, and their right eye the right side, two pictures sufficiently dissimilar t Who has not noticed, as well as Dr. Smith, that when they look at any thin, flat body, such as a thin book, they see both sides of it—the left eye only the left side of it, and the right eye only the right side, while the back, or the part nearest the face, is seen by each eye, and both the sides and the back by both the eyes? What student of perspective is there—master or pupil, male or female—who does not know, as certainly as he knows his alphabet, that the picture of a chair or table, or anything else, drawn from one point of sight, or as seen by one eye placed in that point, is necessarily dissimilar to another drawing of the same object taken from another point of sight, or as seen by the other eye placed in a point 2^ inches distant from the first? If such a person is to be found, we might then admit that the dissimilarity of the pictures in each eye was not known to every student of perspective.1

1 Treatise on Optics, p. 171; Fee also sect. 64. p. 113.

- Treatise on the Et/e, vol i. p. 412, Plate B, Fig. 37.

B

Such was the state of our knowledge of binocular vision when two individuals, Mr. Wheatstone, and Mr. Elliot, now Teacher of Mathematics in Edinburgh, were directing their attention to the subject. Mr. Wheatstone communicated an important paper on the Physiology of Vision to the British Association at Newcastle in August 1838, and exhibited an instrument called a Stereoscope, by which he united the two dissimilar pictures of solid bodies, the To. erioia, (ta sterea of Aguilonius,) and thus reproduced, as it were, the bodies themselves. Mr. Wheatstone's paper on the subject, which had been previously read at the Royal Society on the 21st of June, was printed in their Transactions for 1838.2

Mr. Elliot was led to the study of binocular vision in consequence of having written an Essay, so early as 1823, for the Class of Logic in the University of Edinburgh, "On the means by which we obtain our knowledge of distances by the Eye." Ever since that date he was familiar with the idea, that the relief of solid bodies seen by the eye was produced by the union of the dissimilar pictures of them in each eye, but he never imagined that this idea was his own, believing that it was known to every student of vision. Previous to or during the year 1834, he had resolved to construct an instrument for uniting two dissimilar pictures, or of constructing a stereoscope; but he delayed doing this till the year 1839, when he was requested to prepare an original communication for the Polytechnic Society, which had been recently established in Liverpool. He was thus induced to construct the instrument which he had projected, and he exhibited it to his friends, Mr. Richard Adie, optician, and Mr. George Hamilton, lecturer on chemistry in Liverpool, who bear testimony to its existence at that date. This simple stereoscope, without lenses or mirrors, consisted of a wooden box 18 inches long, 7 broad, and 4J deep, and at the bottom of it, or rather its farther end, was placed a slide containing two dissimilar pictures of a landscape as seen by each eye. Photography did not then exist, to enable Mr. Elliot to procure two views of the same scene, as seen by each eye, but he drew the transparency of a landscape with three distances. The first and most remote was the moon and the sky, and a stream of water from which the moon was reflected, the two moons being placed nearly at the distance of the two eyes, or 2^ inches, and the two reflected moons at the same distance. The second distance was marked by an old cross about a hundred feet off; and the third distance by the withered branch of a tree, thirty feet from the observer. In the right-hand picture, one arm of the cross just touched the disc of the moon, while, in the left-hand one, it projected over one-third of the disc The branch of the tree

1 As Mr. Wheatstone hfm*elf describes the dissimilar pictures or drawings as "two different projections of the same object seen from two points of sight, the distance between which is equal to the interval between the eyes of the observer," it is inconceivable on what ground he could imagine himself to be the discoverer of so palpable and notorious a fact as that the pictures of a iiody seen by two eyes— two points of sight, must be dissimilar. 'Phil. Trans., 1838. pp. 371-394.

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