POPULAR SCIENTIFIC LECTURES. 



613 



light in the gallery, provided he gives to his 

 colors the same ratio of brightness as that 

 which actually exists. 



Foi, in fact, iii looking at natural objects, 

 the absolute brightness in which they ap- 

 pear to the eyo varies within very wide 

 limits, according to the intensity of tho light 

 and tho sensitiveness of tho eye. That 

 which is constant is only the ratio of the 

 brightness iu which surfaces of various 

 depth of color appear to us when lighted to 

 the same amount. But this ratio of bright- 

 ness is for us the perception, from which wo 

 form our judgment as to tho lighter or 

 darker color of the bodies we see. Now this 

 ratio can be imitated by the painter without 

 restraint, and in conformity with nature, to 

 evoke in us tho same conception as to the 

 nature of the bodies seen. A truthful imita- 

 tion in thas respect would be attained within 

 the limits in which Fechner' s law holds, if 

 the artist reproduced the fully lighted parts 

 of the objects which he has to represent 

 with pigments, which, with the samo light, 

 were equal to the colors to be represented. 

 This is approximately the case. On the 

 whole, the painter chooses colored pigments 

 which almost exactly reproduce the colors of 

 the bodies represented, especially for objects 

 of no great depth, such as portraits, and 

 which are only darker in the shaded parts. 

 Children begin to paint on this principle 

 they imitate one color by another ; and, in 

 like manner also, nations in which painting 

 has remained in a childish stage. Perfect 

 artistic painting is only reached when we 

 have succeeded in imitating the action of 

 light upon the eye, and not merely the pig- 

 ments ; and only when we look at the object 

 of pictorial representations from this point 

 of view, will it be possible to understand the 

 variations from nature which artists have to 

 make in the choice of their scale cf color 

 and of shade. 



These are, in the first case, due to the cir- 

 cumstance that Fechner' s law only holds for 

 mean degrees of brightness ; wuile, for a 

 brightness which is too high or too low, ap- 

 preciable divergences are met with. 



At both extremes of luminous intensity the 

 eye is less sensitive for differences in light 

 than is required by that law. With a very 

 Btrong light it is dazzled ; that is, its inter- 

 nal activity cannot keep pace with the ex- 

 ternal excitations ; the nerves are too soon 

 tired. Very bright objects appear almost 

 always to be equally bright even wnen there 

 are, in fact, material diff erences in their lu- 

 minous intensity. The light at tho edge of 

 the sun is only about half as bright as that at 

 the centre, yet none of you will ha^e noticed 

 that, if you have not looked through colored 

 glasses, which reduce the brightness to a con- 

 venient extent. With a weak light the eye is 

 also less sensitive, but from the opposite 

 reason. If a body is so feebly illuminated 

 that we scarcely perceive it, we shall not bo 

 able to perceive that its brightness is les- 

 sened by a shadow by the one hundredth or 

 even by a tenth. 



It follows from this, that, with moderate 

 illumination, darker objects become more 

 like the darkest objects, while with greater 

 illumination brighter objects, become more 

 like the brightest than should be the case in 

 accordance with Fechner' s law, which holds 

 for mean degrees of illumination. From 

 this results, what, for painting, is an ex- 

 tremely characteristic difference between 

 the impression of very powerful and very 

 feeble illumination. 



When painters wish to represent glowing 

 sunshine, they make all objects almost 

 equally bright, and thus produce with their 

 moderately bright colors the impression 

 which the sun's glow makes upon the daz- 

 zled eye of the observer. If, on the contrary, 

 they wish to represent moonshine, they only 

 indicate the very brightest objects, particu- 

 larly the reflection of moonlight on shining 

 surfaces, and keep everything so dark as to 

 be almost unrecognizable ; thafc is to say, 

 they make . all dark objects more like the 

 deepest dark which they can produce with 

 their colors, than should be the case in ac- 

 cordance with the true ratio of the luminos- 

 ities. In both cases they express, by their 

 gradation of the lights, the insenritiveneaa of 

 the eye for differences of too brieht or too 

 feeble lights. If they could emplo] the color 

 of the dazzling brightness of full RUP shine, 

 or of the actual dimness of moonlight, *hey 

 would not need to represent the gradatrou of 

 light in their picture other than it is in na- 

 ture ; the picture would then make tho 

 same impression on the eye as is produced 

 by equal degrees of brightness of actual ob- 

 jects. The alteration in the scale of shade 

 which has been described is necessary be- 

 cause the colors of the picture are seen iu 

 the mean brightness of a moderately lighted 

 room, for which Fechner' s law holds ; and 

 therewith objects are to be represented 

 whose brightness is beyond the limits of this 

 law. 



We find that tho older masters, and pre- 

 eminently Rembrandt, employ the same de- 

 viation, which corresponds to that actually 

 seen in moonlight landscapes ; and this in 

 cases in which it is by no means wished to 

 produce the impression of moonshine, or of 

 a similar feeble light. The brightest parts 

 of the objects are given in these pictures in 

 bright, luminous yellowish colors ; but the 

 shades toward the black are made very mark- 

 ed, so that the darker objects are almost lost 

 ia an impermeable darkness. But this dark- 

 ness is covered with the yellowish haze of 

 powerfully lighted aerial masses, so that, not- 

 withstanding their darkness, these pictures 

 give the impression of sunlight, nud tho 

 very marked gradation of tho shadows, the 

 contours of tho faces and figures, are made 

 extremely prominent. Tho deviation from 

 strict truth to nature is very remarkable in 

 this shading, and yet these pictures give 

 particularly bright and vivid aspects of the 

 objects. Hence they are of particular inter- 

 est for understanding the principles of pic- 

 torial illumination. 



