616 



POPULAR SCIENTIFIC LECTURES. 



not wanting ; only tbey are shadowy in tlieir 

 contours, and of very short duration. 



If a red surface be laid upon a gray ground, 

 and if we look from the red over the edge 

 toward the gray, the edges of the gray will 

 ueem as if struck by such an after-image of 

 red, and will seem to be of a faint, bluish 

 green. But as the after-image rapidly dis- 

 appears it is mostly only those parts of the 

 gray which nro nearest the red which show 

 the change in a marked degree. 



This also is a phenomenon which is pro- 

 duced more strongly by bright light and 

 brilliant, saturated colors than by fainter 

 light and duller colors. The artist, how- 

 ever, works for the most part with the latter. 

 He produces most of his tints by mixture ; 

 ach mixed pigment is, however, grayer and 

 duller than the pure color of which it is 

 mixed, and eveu the few pigments of a 

 highly saturated shade which oil-painting 

 can employ, are comparatively dark. The 

 pigments employed in water-colors and col- 

 ored chalks are again comparatively white. 

 Hence such bright contrasts, as are observed 

 in strongly colored and strongly lighted ob- 

 jects in nature, cannot be expected from their 

 representation in the picture. If, therefore, 

 arith the pigments at his command, the artist 

 vishes to reproduce the impression which 

 objects give, as strikingly as possible, he 

 _mst paint tho contrasts which they pro- 

 duce. If the colors on the picture are as 

 brilliant and luminous as in the actual ob- 

 jects, tho contrasts in the former case would 

 produce themselves as spontaneously as in 

 the latter. Here, also, subjective phenomena 

 of the eye must be objectively introduced into 

 the picture, because the scale of color and of 

 brightness is different upon the latter. 



With a little attention you will see that 

 painters and draughtsmen generally make a 

 plain, uniformly lighted nurface brighter, 

 where it is close to a dark object, and darker, 

 where it i:-? near a light object. You will find 

 that uniform gray surfaces are given a yel- 

 lowish tint at the edye where there is a back- 

 ground of blue, and a rose-red tint where 

 they impinge on green, provided that none 

 of the light collected from the blue or green 

 can fall upon the gray. Where the sun's 

 rays passing through the green leafy shade of 

 trees strike against the ground, they appear 

 to the eye, tired with looking at the predomi- 

 nant green, of a rose-red tint ; the whole 

 daylight, entering through a slit, appears 

 blue, compared with reddish-yellow candle- 

 light. In this way they are represented by 

 the painter, since the colors of his pictures 

 are not bright enough to reproduce the con- 

 trast without such help. 



To tiuj series of subjective phenomena, 

 which artists are compelled to represent ob- 

 jectively in their pictures, must be asso- 

 ciated certain phenomena of iivadiation. By 

 this is imderstood cases in which any bright 

 object in the fieH spreads its light or color 

 over the neighborhood. The phenomena 

 are the more marked the brighter is the radi- 

 ating object, and the halo is brightest ia the 





immediate neighborhood o> tlio bright o 

 ject, but diminishes at a greater disumo 

 These phenomena of irradiation nro :no 

 striking around a very bright light on a d:u ] 

 ground. If the view of the flame itself 

 closed by a narrow dark object such as tl: 

 finger, a bright, misty halo appears, whh i 

 covers the whole neighborhood, und, at tl 

 same time, any objects there may be in th 

 dark part of the field of view are seen nioi 

 distinctly. If the flame is partly screened Vi 

 a ruler, this appears jagged where the flan ' 

 projects beyond it. The luminosity in tb 

 neighborhood of the flame is so intense thi 

 its brightness can scarcely be distinguishe 

 from that of the flame itself ; as is the c&f 

 with all bright objects, the flame appeal 

 magnified, and as if spreading over towar 

 the adjacent dark objects. 



The causa of this phenomenon is quit 

 similar to that of aerial pespective. It is du< 

 to a diffusion of light which arises fron 

 the passage of light through dull media, ex 

 cepting that for the phenomena of aerial per 

 spective the turbidity is to be sought in tht 

 air in front of the eye, while for true phc 

 nomena of irradiation it is to be sought in tht 

 transparent media of the eye. When ever 

 the healthiest human eye is examined \>\ 

 powerful light, the best being a pencil o 

 sunlight concentrated on the side by a con 

 densing lens, it is seen that the sclerotici 

 and crystalline lens are not perfectly clear 

 If strongly illuminated, they both appeal 

 whitish and as if rendered turbid by a tin 

 mist. Both are, in fact, tissues of fibrour 

 structure, and are not therefore so hoinogene 

 ous as a pure liquid or a pure crystal. Ever 

 inequality, however small, in the structure 

 of a transparent body can, however, reflect 

 some of the incident light that is, can 

 diffuse it in all directions.* 



The phenomena of irradiation alr>o oocm 

 with moderate degrees of brightness. A darl; 

 aperture in a sheet of paper illuminated b; 

 the sun, or a small, dark object on a colored 

 glass plate which is held against the clear 

 sky, appear as if the color of the adjacen': 

 surfaco were diffused over them. 



Hence the phenomena of irradiation an 

 very similar to those which produce tin 

 opacity of the air. The only essential differ- 

 ence lies in this, that the opacity by lumin 

 ous air is stronger before distant object 

 which have a greater mass of air in front oi 

 them than before near ones ; while irradia- 

 tion in the eyes sheds its halo uniformly ovei 

 near and over distant objects. 



Irradiation also belongs to the subjective 

 phenomena of the eye which the artist, repre- 

 sents objectively, because painted lights and 

 painted sunlight are not bright enough t? 

 produce a distinct irradiation in the eye cf 

 the observer. 



The representation which the painter ha 



* I cli*re?ard here tho view thfit irrndmtion in tlia 

 ovu dependu on a diffusion of lh excitation in the 

 substance of th nerve*, as this appears to me too 

 hypothetic," I. Moreover, we are here concerned 

 with, tho phc-noineua and not \vitli their cause. 



