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,[19] 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? 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 2½ 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.[20] [18] _Treatise on Optics_, p. 171; see also sect. 64, p. 113. [19] _Treatise on the Eye_, vol i. p. 412, Plate 5, Fig. 37. [20] As Mr. Wheatstone himself 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 body seen by two eyes—_two points of sight_, must be dissimilar. 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 =τὰ στέρεα=, (_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.[21] [21] _Phil. Trans._, 1838, pp. 371-394.