Chap. CXXV.—How to give the proper Dimension to Objects in Painting.
In order to give the appearance of the natural size, if the piece be small (as miniatures), the
be the case, if the angle of the visual rays were not too big for the first object. Whoever is at all acquainted with optics, need not be told, that the visual rays commence in a single point in the centre, or nearly the centre of each eye, and continue diverging. But, in using both eyes, the visual rays proceed not from one and the same centre, but from a different centre in each eye, and intersecting each other, as they do a little before passing the first object, they become together broader than the extent of the first object, and consequently give a view of part of the second. On the contrary, in using but one eye, the visual rays proceed but from one centre; and as, therefore, there cannot be any intersection, the visual rays, when they reach the first object, are not broader than the first object, and the second is completely hidden. Properly speaking, therefore, in using both eyes we introduce more than one point of sight, which renders the perspective false in the painting; but in using one eye only, there can be, as there ought, but one point of sight. There is, however, this difference between viewing real objects and those represented in painting, that in looking at the former, whether we use one or both eyes, the objects, by being actually detached from the back ground, admit the visual rays to strike on them, so as to form a correct perspective, from whatever point they are viewed, and the eye accordingly forms a perspective of its own; but in viewing the latter, there is no possibility of varying the perspective; and, unless the picture is seen precisely under the same angle as it was painted under, the perspective in all other views must be false. This is observable in the perspective views painted for scenes at the playhouse. If the beholder is seated in the central line of the house, whether in the boxes or pit, the perspective is correct; but, in proportion as he is placed at a greater or less distance to the right or left of that line, the perspective appears to him more or less faulty. And hence arises the necessity of using but one eye in viewing a painting, in order thereby to reduce it to one point of sight.
