250 THE POPULAR SCIENCE MONTHLY. 



we must look at it with one eye ; for, in looking with two eyes, the 

 duplication of the visual lines that define the position of each point 

 as it is perceived, must cause more or less of the impression of the 

 really plain surface to persist. In this case the corporeal projection 

 becomes confused with the superficial conception, and is more or less 

 modified by it. The effect of perspective is also heightened by look- 

 ing at the painting through a tube, by means of which it is abstracted 

 from the frame and from its surroundings. 



We observe, also, in looking at a picture, the curious phenomenon 

 that, whatever position we may take toward it, it always appears 

 projected, but in aspects which are varied not only in respect to the 

 absolute position of its constituent parts, but also in respect to their 

 mutual relations with one another, to such an extent that from an ex- 

 tremely unfavorable position it appears distorted. 



Whenever an artist can not produce the perspective effects he de- 

 sires in a picture drawn with absolute correctness, he does not hesitate 

 to exaggerate the perspective if he thinks he can thereby enforce a 

 better conception of his design. lie represents distant objects on a 

 smaller scale than strict accuracy demands, and causes us, by uncon- 

 sciously taking account of the reduction, to see them farther off than 

 we otherwise would do ; and he knows how to call our imagination to 

 his help in other ways. 



Although the application of perspective devices is usually enough 

 for the purpose, the projection of the design is heightened to a con- 

 siderable degree, facilitated and brought nearer the appearance of 

 reality by the introduction of effects of light and shade. This also 

 is borrowed from Nature. The shading may, indeed, sometimes define 

 and fix the effect which mere perception fails to convey positively. A 

 square with two diagonals drawn across it may mean simply that, or it 

 may be intended to represent a pyramid. If proper shading is added, 

 the figure is made to mean a pyramid unmistakably. 



If we assume that the light is evenly diffused through any space, 

 then, other conditions being equal, those things in the space which are 

 nearer to us will be clearer in appearance in a proportion determined 

 by the rule that the intensity of light diminishes as the square of the 

 distance increases. A point twice as far from the eye as another ap- 

 pears four times as obscure. This normal diminution of intensity is 

 augmented by the darkening which objects undergo in consequence of 

 the interposition of strata of air, or by the effect of "air-perspective," 

 as it is called, in distinction from linear perspective. The painter must 

 take both of these phenomena into account. By the application of a 

 suitable coloring, the effect of air-perspective may be produced in so 

 striking a degree that objects may, by means of it, be made to appear 

 wonderfully remote. This effect, also, is dependent upon our uncon- 

 scious application of knowledge we have gained from previous expe- 

 rience. 



