84 ON THE KELATION OF OPTICS TO PAINTING. 



sion, which impresses itself without fatigue on the 

 observer. 



This direct intelligibility is again the preliminary 

 condition for an undisturbed, and vivid action of the 

 picture on the feelings and mood of the observer. 



The subordinate methods of expressing depth which 

 have been referred to, depend in the first place on per- 

 spective. Nearer objects partially conceal more distant 

 ones, but can never themselves be concealed by the 

 latter. If therefore the painter skilfully groups his ob- 

 jects, so that the feature in question comes into play, 

 this gives at once a very certain gradation of far and 

 near. This mutual concealment may even preponderate 

 over the binocular perception of depth, if stereoscopic 

 pictures are intentionally produced in which each coun- 

 teracts the other. Moreover, in bodies of regular or of 

 known form, the forms of perspective projection are for 

 the most part characteristic for the depth of the object. 

 If we look at houses, or other results of man's artistic 

 activity, we know at the outset that the forms are for the 

 most part plane surfaces at right angles to each other, 

 with occasional circular or even spheroidal surfaces. And 

 in fact, when we know so much, a correct perspective 

 drawing is sufficient to produce the whole shape of the 

 body. This is also the case with the figures of men and 

 animals which are familiar to us, and whose forms 

 moreover show two symmetrical halves. The best per- 



