APPEARANCE AND REALITY IN PICTURES. 249 



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. 



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APPEARANCE AND EEALITY IN PICTURES. 



Bt Dr. EUGEN DREHER. 



IN 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 organi- 

 zation of the visual apparatus, that amounts in effect to the transfor- 

 mation 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 contem- 

 plating 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 projec- 

 tion. 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, 



