Galen, therefore, and the Neapolitan philosopher, who has employed a more distinct diagram, certainly knew and adopted the fundamental principle of the stereoscope; and nothing more was required, for producing pictures in full relief, than a simple instrument for uniting HF and GE, the right and left hand dissimilar pictures of the column. [Illustration: FIG. 2.] In the treatise on painting which he left behind him in MS.,[4] Leonardo da Vinci has made a distinct reference to the dissimilarity of the pictures seen by each eye as the reason why “a painting, though conducted with the greatest art, and finished to the last perfection, both with regard to its contours, its lights, its shadows, and its colours, can never shew a _relievo_ equal to that of the natural objects, unless these be viewed at a distance and with a single eye,”[5] which he thus demonstrates. “If an object C be viewed by a single eye at A, all objects in the space behind it—included, as it were, in a shadow ECF, cast by a candle at A—are invisible to an eye at A; but when the other eye at B is opened, part of these objects become visible to it; those only being hid from both eyes that are included, as it were, in the double shadow CD, cast by two lights at A and B and terminated in D; the angular space EDG, beyond D, being always visible to both eyes. And the hidden space CD is so much the shorter as the object C is smaller and nearer to the eyes. Thus he observes that the object C, seen with both eyes, becomes, as it were, transparent, according to the usual definition of a transparent thing, namely, that which hides nothing beyond it. But this cannot happen when an object, whose breadth is bigger than that of the pupil, is viewed by a single eye. The truth of this observation is, therefore, evident, because a painted figure intercepts all the space behind its apparent place, so as to preclude the eyes from the sight of every part of the imaginary ground behind it. Hence,” continues Dr. Smith, “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_.” [4] _Trattata della Pictura, Scultura, ed Architettura._ Milan, 1584. [5] Dr. Smith’s _Compleat System of Opticks_, vol. ii., Remarks, pp. 41 and 244.