Page:Popular Science Monthly Volume 31.djvu/261

Rh milling process for the inferior quality of sand, some of which goes into mortar for specially fine and durable wall-building. The rail-roads use large quantities of it in the construction of retaining-walls for embankments. And so all grades of the sand are utilized.



N the contemplation of the creations of the painter, the mind is stimulated to a degree of activity which the enjoyment of no other form of art-work can induce. A mental operation is provoked of which we are hardly conscious, and which some have attributed to the organization of the visual apparatus, that amounts in effect to the transformation of the flat surfaces of the picture into the appearance of a body or a group standing out or receding in relief. The inquiry as to how the painter can invoke this illusion is usually answered by saying that he knows how to represent objects in perspective; that is, that he is able to arrange the lines of the picture—except that the image is not reversed—so that the adjustment shall correspond essentially with that of the image which is cast upon the photographer's screen or upon the retina of the eye. The process, unperceived and instantaneous in the case of simple objects, by which such representations are given bodily projection, may be followed out in its gradual development in contemplating pictures of a more complex character, as, for instance, a view of the interior of a grand cathedral. Without any change taking place in the image on the retina, the individual objects are gradually lifted one from another; those represented as in the background appear to become larger but at the same time obscure, and those in the fore-ground to grow smaller but more sharply defined. Thus the size we attribute to the objects depends upon the distance we assign to them as well as on the visual angle they subtend. There are, according to this, unconscious processes that fit us for seeing plane surfaces as bodies, provided the picture furnishes suitable points to which our conceptions of corporeal projection may attach themselves. To see a perspective representation of a cube, I must have remaining within me a conception of a cube already acquired by the exercise of my senses; and, without such an acquired conception, the picture would still be to me only a picture or a planimetric feature, without projection. Hence we find that, in looking at pictures, unconscious representations intrude upon the primary conceptions, and change them into secondary ones, by means of which a surface is made to look like a body having projection. This process occurs in all monocular vision, when we interpret the flat retinal image corporeally.

To obtain a proper appreciation of a perspectively correct picture, 