Aii sci:;:;i*ir:c LIXTUKES. 



ens 



or shade, and in such a manner that the 

 original color appears darker by the proxim- 

 ity of a brighter shade, and brighter by that 

 of a darker shade ; while by a color of a 

 different kind it tends toward the comple- 

 mentary tint. 



The phenomena of contrast are very va- 

 rious, and depend on different causes. One 

 class, Chevreul's ftirrmlianwus Contrast, is inde- 

 pendent of the motions of the eyes, and oc- 

 curs with surfaces where there are very- 

 slight differences in color and shade. This 

 contrast appears both on the picture and in 

 actual objects, and is well known to painters. 

 Their mixtures of colors on the palette often 

 appear quite different to what they are on 

 the picture. The changes of color which are 

 here met with are often very striking ; I will 

 not, however, enter upon them, for they pro- 

 duce no divergence between the picture and 

 reality. 



The second class of phenomena of con- 

 trast, and one which, for us, is mora impor- 

 tant, is met with in changes of direction of 

 the glance, and more especially between sur 

 faces, in which there are great differences of 

 shade and of color. As the eye glides over 

 bright and dark, or colored objects and sur- 

 fiuvs, the impression of each color changes, 

 tor it is depicted on portions of the retina 

 which directly before were struck by other 

 colors and lights, and were therefore changed 

 in their sensitiveness to an impression. This 

 kind of contrast is therefore essentially de- 

 pendent on movements of the eye, and has 

 been called by Chevreul, " successive Contrast." 



We have already, seen that the retina is 

 more sensitive in the dark to feeble light 

 than it was before. 13y strong light, on the 

 contrary, it is dulled, and is less sen.iitive to 

 feeble lights which it had before perceived. 

 This latter process is designated as " Fa- 

 tigue" of the retina ; an exhaustion of the 

 capability of the retina by its own activity, 

 just as the muscles by their activity become 

 tired. 



I must here remark that the fatigue of the 

 retina by light does not necessarily extend to 

 tho whole surface ; but when only a small 

 portion of this membrane is struck by a 

 minute, denned picture it can also be locally 

 developed in this part only. 



You must all have observed the dark spots 

 which move about in tho field of vision, 

 when we have been looking for only a short 

 time toward the setting sun, and which 

 physiologists call negative after-images of the 

 sun. They are duo to the fact that only 

 those parts of the retina which are actually 

 struck by the image of the sun in the eye, 

 have become insensitive to a new impres- 

 sion of light. If, with an eye which is thus 

 locally tired, we look toward a uniformly 

 bright surface, such as the sky, tho tired 

 parts of the retina aro more feebly and more 

 darkly affected than the other portions, so 

 that the observer thinks he sees dark spots 

 in the sky, which move about with his sight. 

 We have then in juxtaposition, in the bright 

 parts of the sky, the impression which these 



make upon tho untired parts of the retina, 

 and in the dark spots their action on the 

 tired portions. Objects, bright like the sun, 

 produce negative after-images in tho most 

 striking manner ; but with a little attention 

 they may be seen even after much more 

 moderate impressions of light. A longer 

 time is required in order to develop such an 

 impression, so that it may bo distinctly rec- 

 ognized, and a definite point of the bright 

 object must be fixed, without moving the 

 eye, so that its image may be distinctly 

 formed on tho retina, and only a limited por- 

 tion of the retina be excited and tired, just 

 as in producing sharp photographic portraits 

 the object must bo stationary during the 

 time of exposure in order that its image may 

 not be displaced on the sensitive plate. The 

 after-image in the eyo is, as it were, a photo- 

 graph on the retina, which becomes visible 

 owing to the altered sensitiveness toward 

 fresh light, but only remains stationary for a 

 short time ; it is longer, the more powerful 

 and durable was the action of light 



If the object viewed was colored, for in- 

 stance red paper, the after-image is of the 

 complementary color on a gray ground ; in 

 this case of a bluish green.* Rose-red paper, 

 on the contrary, gives a pure green after- 

 image, green a rose-red, blue a yellow, and 

 yellow a blue. These phenomena, show that 

 in the retina partial fatigue is possible for 

 tha several colors. According to Thoinan 

 Young's hypothesis of tho existence of three 

 systems of fibres in the visual nerves,* of 

 which one set perceives red whatever the 

 kind of irritation, the second green, and the 

 third violet, with green light, only thoso 

 fibres of the retina which arc sensitive to 

 green are powerfully excited and tired. If 

 this same part of the retina is afterward il- 

 luminated with white light, tho sensation of 

 green is enfeebled, while that of red and vio- 

 let is vivid and predominant ; their sum 

 gives tho sensation of purplo, which mixed 

 with tha unchanged white ground forms 

 rose-red. 



In the ordinary way of looking at light and 

 colored objects, we arj not accustomed to 

 fix continuously 0110 and tho same point ; for 

 following with the gazo the play of our at- 

 tentiveness, we ara always turning it to new 

 parts of the object as they happen to interest 

 us. This way of looking, in which the eye 

 is continually moving, an;l therefore the 

 retinal image is also shifting about on tha 

 retina, has moreover the advantage of avoid- 

 ing disturbances of sight, which poworful 

 and continuous after-images would bring 

 with them. Yet here also, after-imago* are 



* In order to nee thin kind of imn','0 as distinctly 

 a" poiuible. it ia desirable tx> avoid all movements of 

 the eye. On n large sheet of dark gray pa|er ft 

 small bltick cross is draws;, the centre of which ia 

 steadily viewed, and a quadrangular i-lieet of paper 

 <>f that C' lor whose after- imitge is to be observed ia 

 slid from the fide, co that one of its corners touches 

 the CIOKS. The cheet is allowed to remain for a min- 

 ute or two. the. rross being steadilv viewed, and it is 

 then drawn suddenly away, without relaxing the 

 view. In place of the sheet removed the after imag 

 appears tUcii on the dark ground. 



