We have quoted this passage, not from its _proving_ that Leonardo da Vinci was acquainted with the fact that each eye, A, B, sees dissimilar pictures of the sphere C, but because it has been referred to by Mr. Wheatstone as the only remark on the subject of binocular vision which he could find “after looking over the works of many authors who might be expected to have made them.” We think it quite clear, however, that the Italian artist knew as well as his commentator Dr. Smith, that each eye, A and B, sees dissimilar parts of the sphere C. It was not his purpose to treat of the binocular pictures of C, but his figure proves their dissimilarity. The subject of binocular vision was successfully studied by Francis Aguillon or Aguilonius,[6] a learned Jesuit, who published his Optics in 1613. In the first book of his work, where he is treating of the vision of solids of all forms, (_de genere illorum quæ τὰ στέρεα [ta sterea] nuncupantur_,) he has some difficulty in explaining, and fails to do it, why the two dissimilar pictures of a solid, seen by each eye, do not, when united, give a confused and imperfect view of it. This discussion is appended to the demonstration of the theorem, “that when an object is seen with two eyes, two optical pyramids are formed whose common base is the object itself, and whose vertices are in the eyes,”[7] and is as follows:— [6] _Opticorum Libri Sex Philosophis juxta ac Mathematicis utiles._ Folio. Antverpiæ, 1613. [7] In FIG. 1, AHF is the optical pyramid seen by the eye A, and BGE the optical pyramid seen by the eye B. “When one object is seen with two eyes, the angles at the vertices of the optical pyramids (namely, HAF, GBE, Fig. 1) are not always equal, for beside the direct view in which the pyramids ought to be equal, into whatever direction both eyes are turned, they receive pictures of the object under inequal angles, the greatest of which is that which is terminated at the nearer eye, and the lesser that which regards the remoter eye. This, I think, is perfectly evident; but I consider it as worthy of admiration, how it happens that bodies seen by both eyes are not all confused and shapeless, though we view them by the optical axes fixed on the bodies themselves. For greater bodies, seen under greater angles, appear lesser bodies under lesser angles. If, therefore, one and the same body which is in reality greater with one eye, is seen less on account of the inequality of the angles in which the pyramids are terminated, (namely, HAF, GBE,[8]) the body itself must assuredly be seen greater or less at the same time, and to the same person that views it; and, therefore, since the images in each eye are dissimilar (_minime sibi congruunt_) the representation of the object must appear confused and disturbed (_confusa ac perturbata_) to the primary sense.” [8] These angles are equal in this diagram and in the vision of a sphere, but they are inequal in other bodies.