THE SENSE OF SIGHT 189 



to the colors of the background is an enigma which it is not at present possi- 

 ble fully to solve. 



A second series of interesting facts consists in the so-called secondary images. 

 When an impression of light strikes the retina, the sensation thereby generated 

 does not vanish instantaneously with the cessation of the impression from 

 without, but subsists yet a short time longer. A number of familiar phenomena 

 admit of being referred to this fact. When a wheel turu'^ very rapidly we no 

 longer distinguish the single spokes — it appears as a full disk ; when we whirl 

 a glowing coal rapidly around we do not see the coal gradually advancing as a 

 *night point ; it presents a closed circle of light ; that is to say, we see the coal 

 simultaneously at all points of its progress. Whence is this ? It results simply 

 from the persistence of the impression. The coal produces at each point of its 

 course a sensation of light which endures for a moment, while the coal itself has 

 already moved forward. If we imagine the coal at the uppermost point of its 

 circuit, the sensation generated at this point still endures, that is, we still see 

 the coal at this uppermost point, while it has already moved a space further, 

 and has generated sensations from the following points, which again continue a 

 moment beyond the time of its presence at any given point. With a certain 

 velocity of rotation, it will follow that the sensation produced at the first point 

 still persists when the coal has performed its whole circuit, again arrived at the 

 uppermost point, and there generated the impression anew ; and since the same 

 is the case with each successive point, the image presented to our eye is, of 

 course, that of a circle of light. The same holds good Avith regard to the re- 

 volving wheel. A further interesting example is the folloAving : We have seen 

 that white light is compounded of seven colors — those, namely, of the rainbow; 

 in other words, a ray of light Avhich generates in the eye the sensation of a 

 white color is composed of seven luminous undulations, differing in the vibratory 

 velocity of their ethereal particles, each of which undulations, when it strikes 

 the eye separately, generates the sensation of a peculiar color. Now, if we take 

 a disk, divide its surface from the centre outward into seven compartments, color 

 these successively with the tints of the rainbow, and in the same order, the first 

 red, the second orange, &c., and then cause the disk to revolve very rapidly, it 

 will appear to the eye to be white, or, at least, gray. After what has preceded, the 

 explanation is simple. Through the rapid movement of the disk the several 

 colored rays strike so quickly one after another upon the portion of the retina 

 on which the image is traced, that the impression of the first ray is not yet ex- 

 tinct when the remaining six have already struck upon the same sensitive 

 points, whence the retina is simultaneously excited by the seven rays, and a 

 perception of whiteness is the consequence. From this simple persistence of 

 the original impression we should distinguish the secondary images which some- 

 times occur in a narrower sense of the word ; phenomena which, like the former, 

 are familiar to every one from experience. Wlio has not looked upon the set- 

 ting sun, and then perceived the persistent after-images which occupy the field 

 of vision wherever the eyes may be turned, and vanish not even when they are 

 closed ; which, on the contrary, then appear vividly and arrayed in different 

 colors, red, blue, &c., while to the open eye, directed, for instance, towards the 

 bright sky, they seem projected as a dark object on the back ground. By at- 

 tentive obseivation and some practice many analogous phenomena of great di- 

 versity and interest may be observed. We confine our exemplification to an 

 easily repeated experiment. If we place a red wafer (or red paper) on a white 

 ground and observe it attentively for some time, then suddenly turn the sight 

 laterally to the white surface, we shall plainly perceive an after-image of the 

 wafer of a green tint, which follows the eye when directed to different parts of 

 the back ground. If the object thus fixedly observed be blue, the secondary 

 image will be yellow ; in a word, it will be always the so-called contrasted or 

 complementary color which presents itself in this image, and always darker 



