610 



POPULAR SCIENTIFIC LECTURES. 



whereby the ethereal oils :ml resins dis- 

 solved by the latter, separate ont and pro- 

 duce the turbidity. Excessively line blue 

 elouds, bluer even than the air, may bo pro- 

 duced, as Tyndall has observed, when the 

 nun's light is allowed to exert its decompos- 

 ing action on the vapors of certain carbon 

 compounds. Goethe called attention to the 

 universality of this phenomenon, and en- 

 deavored to base upon it his theory of 

 color. 



By aerial perspective we understand the 

 artistic representation of aerial turbidity ; 

 for the greater or less predominance of the 

 aerial color above the color of the objects, 

 shows their varying distance very definitely ; 

 and landscapes more especially acquire the ap- 

 pearance of depth. According to the weather 

 the turbidity of the air mtvy be greater or less, 

 more white or more blue. Very clear air, as 

 sometimes met with after continued ruin, 

 makes the distant mountains appear small 

 and near ; whereas wheii the air contains 

 more vapor, they appear large and distant. 



This latter is decidedly better for the land- 

 scape painter, and the high transparent land- 

 scapes of mountainous regions, which so 

 often lead the Alpine climber to underesti- 

 mate the distance and the magnitude of the 

 mountain-tops before him, are also difficult 

 to turn to account in a picturesque manner. 

 Views from the valleys, and from seas and 

 plains in which the aerial light is faintly but 

 markedly developed, are far better ; not only 

 do they allow the various distances and mag- 

 nitudes of what is seen to stand out, but they 

 are on the other hand favorable to the artis- 

 tic unity of coloration. 



Although aerial color is most distinct in 

 the greater depths of landscape, it is not en- 

 tirely wanting in front of the near objects of 

 a room. "What is seen to bo isolated and 

 well defined, when sunlight passes into n 

 dark room through a hole in the shutter, is 

 also not quite wanting when the whole room 

 is lighted. Here, also, the aerial lighting 

 must stand out against the background, and 

 must somewhat deaden the colors in compari- 

 son with those of nearer objects ; and these 

 differences, also, although far more delicate 

 than against the background of a landscape, 

 are important for the historical, genre, or por- 

 trait painter ; and when they are carefully 

 observed and imitated, they greatly heighten 

 the distinctness of his representation. 



IL SHACK. 



The circumstances which we have hitherto 

 discussed indicate a profound difference, 

 and one which is exceedingly important for 

 the perception of solid form, between the 

 visual image which our eyes give, when we 

 Ktand before objects, and that which the pic- 

 ture givtM. The choice of the objects to be 

 represented in pictiires is thereby at once 

 much restricted. Artists are well aware that 

 there is much which cannot bo represented 

 by the means at their disposal. Part of thoir 

 artistic skill consists la Uu> fact that bv a 



suitable grouping, position, and turn 

 objects, by a suitable choice of the point o 

 view, and by the mode of lighting, they learn 

 to overcome the unfavorable conditions whicl 

 are imposed on them in this respect. 



It might at first sight appear that of the 

 reqiiisite tmth to nature of ft picture, sr 

 much would remain that, seen from th 

 proper point of view, it would at least produc 

 the same distribution of light, color, and 

 shadow in its field of view, and would pro 

 duce in the interior of the eye exactly th 

 same image on the retina as the Abject rep 

 resented would do if we had it actually be 

 fore us, and looked at it from a definite 

 fixed point of view. It might seem to be ai 

 object of pictorial skill to aim at prodxioing 

 under the given limitations, the saim effcc 

 as is produced by the object itself. 



If we proceed to examine whether, and ho\ 

 far, painting can satisf y such a condition, w 

 come upon diliiculties before which we shoulc 

 perhaps shrink, if we did not know that the 

 had been already overcome. 



Let us begin with the simplest case ; wit 

 the quantitative relations between luminou 

 intensities. If the artist is to imitate exnctl; 

 the impression which the object produces or 

 our eye, he ought to be able to dispose o 

 brightness and dar!iness equal to that whic. 

 nattire offers. But of this there can be n 

 idea. Let mo givo a case in point. Le 

 there ba, in a picture-gallery, a desert-Kcenc- 

 in which a pjf>cession of Bedotiins, shroude 

 in white, and of dark negroes, marche 

 under the burning sunshine ; close to it 

 bluish moonlight scene, where the moon i 

 reflected in the water, and groups of trees, 

 and human forms, are seen to be faintly in- 

 dicated in the darkness. You know from 

 experience that both pictures, if they are we; 

 done, can produce with surprising vividiiei 

 the representation of their objects ; and yt 

 in both pictures the brightest parts are prc 

 duced with the same white-lead, which i 

 but slightly altered by admixtures ; whil 

 the darkest parts are produced with the sain 

 black. Both being hung on the same v/a 

 share the same light, and the brightest t 

 well as the darkest parts of the two source! 

 differ as concerns the degree of their brigh 

 ness. 



How is it, however, with the actual degroo 

 of brightness represented ? Tho relatioi 

 between the brightness of the sun's light an 

 that of the moon was measured by Wollas 

 ton, who compared their intensities wit' 

 that of the light of candles of the same rn: 

 terial. He thus found that the luminosity o 

 the sun is 800,000 times that of the brighter 

 light of a full moon. 



An opaque body, which is lighted from an; 

 source whatever, can. even in the most f;t 

 vorable case, only emit as much light as fall 

 upon it. Yet, from Lambert's observations 

 even the whitest bodies only reflect abon 

 two fifths of the incident light. The sun 

 rays, which proceed parallel from the SUH 

 whose diameter is 83,000 miles, when tht; 

 reach us. are distributed uniformly over 



