S14 



POPULAR SCIENTIFIC LECTURES. 



In order to explain these actions we must, 

 I think, consider that while Fechner's law is 

 approximately correct for those mean lights 

 which lire agreeable to the eye, the devia- 

 tions which arc so marked, for too high or 

 too low lights, are not without some influence 

 in the region of the middle lights. We have 

 to observe more closely in order to perceive 

 this influence. It is found, in fact, that 

 when the very finest differences of shade are 

 reproduced on a rotating disk, they are only 

 visible by a light which about corresponds to 

 the illumination of a white paper on a bright 

 day, which is lighted by the light of the sky, 

 but is not diroctly struck by the sun. With 

 such a light, shades of -y^ or T ^ 5 of the total 

 intensity can bo recognized. The light in 

 which pictures are looked ut is, on the con- 

 trary, much feebler ; and if we are to retain 

 the same distinctness of the finest shadows 

 and of the modelling of the contours which 

 it produces, the gradations of shade in the 

 picture must bo somewhat stronger than cor- 

 responds to the exact luminous intensities. 

 The darkest objects of the picture thereby 

 become unnaturally dark, which is however 

 not detrimental to the object of the artist if 

 the attention of the observer is to be directed 

 to the brighter parts. The great artistic 

 effectiveness of this manner shows us that 

 the chief emphasis is to be laid on imitating 

 difference of brightness and not on absolute 

 brightness ; and that the greatest differences 

 in this latter respect can be borne without 

 perceptible incongruity, if only their grada- 

 tions are imitated with expression. 



m. COLOU. 



With these divergences in brightness are 

 connected certain divergences in color, 

 which, physiologically, are caused by the 

 fact that th e scale of sensitiveness is different 

 for different colors. The strength of the 

 sensation produced by light of a particular 

 color, and for a given intensity of light, de- 

 pends altogether on the special reaction of 

 that complex of nerves which are set in 

 operation by the action of the light in ques- 

 tion. Now all our sensations of color are 

 admixtures of three simple sensations ; 

 namely, of red, green, and violet, * which, by 

 a not improbable supposition of Thomas 

 Young, can be apprehended quite independ- 

 ently of each other by three different systems 

 of nerve-fibres. To this independence of the 

 different sensations of color corresponds 

 their independence in the gradation of in- 

 tensity. Recent measurements! have shown 

 that the sensitiveness of our eye for feeble 

 shadows is greatest in the blue and least in 

 the red. A difference of -j^ to -^-g of the 

 intensity can be observed in the blue, and 

 with an untired eye of -fa in the red ; or 

 when the color is dimmed by being looked 

 at for a long time, a difference of ^ to -V. 



Red, therefore, acts as a color toward whose 

 shades the eye is relatively less sensitive 

 than toward that of blue. In agreement with 

 this, the impression of glare, as the intensity 

 increases, is feebler in red than in blue. Ac- 



cording to an observation of Dovo, if a bio 

 and a red paper be chosen which appear o 

 equal brightness xinder a mean degree c. 

 wnite light, as the light is made much dim- 

 mer the blue appears brighter, and as tin 

 light is much strengthened, the rod. I my- 

 self have found that the same differences ar> 

 seen, and even in a more striking manner, in 

 the red and violet spectral colors, and, wheu 

 their intensity is increased only moderately, 

 by the same fraction for both. 



Now the impression of white is made up 

 of the impressions which the individual 

 spectral colors make on our eye. If we in- 

 crease the brightness of white, the strength 

 of the sensation for the red and yellow rays 

 will relatively be more increased than that 

 for the blue and violet. In bright white, 

 therefore, the former will produce a rela- 

 tively stronger impression than the latter ; 

 in dull white the blue and bluish colors will 

 have this effect. Very bright white appears 

 therefore yellowish, and dull white appear* 

 bluish. In our ordinary way of looking at 

 the objects about us we are not so readily 

 conscious of this ; for the direct comparison 

 of colors of very different shade is difficult, 

 and wo are accustomed to see in this altera- 

 tion in the white the result of different illu- 

 mination of one and the same white object, 

 so that in judging pigment-colors we havo 

 learned to eliminate the influence of bright- 

 ness. 



]f, however, to the painter is put the prob- 

 lem of imitating, with faint colors, white ir- 

 radiated by the sun, he can attain a hi^h de- 

 gree of resemblance ; for by an admixture of 

 yellow in his white he makes this color pre- 

 ponderate just as it would preponderate in 

 actual bright light, owing to the impression 

 on the nerves. It is the same impression as 

 that produced if we lock at a clouded land- 

 scape through a yellow glass, and thereby 

 give it the appearance of a sunny light. The 

 artist will, on the contrary, give a bluish 

 tint to moonlight, that is, a faint white ; for 

 the colors on the picture must, as we have 

 seen, be far brighter than the color to bo rep- 

 resented. In moonshine scarcely any other 

 color can be recognized than blue ; the blue 

 starry sky or blue colors may still appear dis- 

 tinctly colored, while yellow and red can only 

 be seen as obscurations of the general bluish 

 white or gray. 



I Vfill again remind you that theso changes 

 of color would not bo necessary if the artist 

 had at his disposal colors of the same bright- 

 ness, or the same faintness, as are actually 

 shown by the bodies irradiated by the sun or 

 by the moon. 



The change of color, like the scale of 

 shade, previously discussed, is a subjective 

 action which the artist must represent objec- 

 tively on his canvas, since moderately bright 

 colors cannot produce them. 



We observe something quite similar in re-, 

 gard to the phenomena of Contrast. By this 

 term we understand cases in which the color 

 or brightness of a surface appears changed 

 by the proximity of a mass of another colox 





