Page:Essays on the active powers of the human mind; An inquiry into the human mind on the principles of common sense; and An essay on quantity.djvu/575

 OF SEEING. 555

eyes. Such mistakes are often discovered in snuffing a candle, in threading a needle, or in filling a tea- cup.

When a picture is seen with both eyes, and at no great dis- tance, the representation appears not so natural as when it is seen only with one. The intention of painting being to deceive the eye, and to make things appear at different distances which in reality are upon the same piece of canvass, this deception is not so easily put upon both eyes as upon one ; because we perceive the distance of visible objects more exactly and determinately with two eyes than with one. If the shading and relief be exe- cuted in the best manner, the picture may have almost the same appearance to one eye as the objects themselves would have, but it cannot have the same appearance to both. This is not the fault of the artist, but an unavoidable imperfection in the art. And it is owing to what we just now observed, that the percep- tion we have of the distance of objects by one eye is more un- certain, and more liable to deception, than that which we have by both.

[The great impediment, and I think the only invincible impe- diment, to that agreeable deception of the eye which the painter aims at, is the perception which we have of the distance of visible objects from the eye, partly (1) by means of the conformation of the eye, but chiefly (*) by means of the inclination of the optic axes.] If this perception could be removed, I see no reason why a picture might not be made so perfect as to deceive the eye in reality, and to be mistaken for the original object. Therefore, in order to judge of the merit of a picture, we ought, as much as possible, to exclude these two means of perceiving the dis- tance of the several parts of it.

In order to remove this perception of distance, the connois- seurs in painting use a method which is very proper. They look at the picture with one eye, through a tube which excludes the view of all other objects. By this method, the principal mean whereby we perceive the distance of the object, to wit, the incli- nation of the optic axes, is entirely excluded. I would humbly propose, as an improvement of this method of viewing pictures, that the aperture of the tube next to the eye should be very small. If it is as small as a pin-hole, so much the better, pro- viding there be light enough to see the picture clearly. The reason of this proposal is, that when we look at an object through a small aperture, it will be seen distinctly whether the conform- ation of the eye be adapted to its distance or not, and we have no mean left to judge of the distance, but the light and colouring, which are in the painter's power. If, therefore, the artist per- forms his part properly, the picture will by this method affect the eye in the same manner that the object represented would do, which is the perfection of this art.