190 ON THE SENSES. 



than the bright ground on which we observe it. If this ground be coloi'ed, the 

 color of the secondary image varies in different ways which we need not spe- 

 cially describe ; a blue wafer, for instance, observed on a yellow ground yields, 

 when the eye is turned to a white surface, a yellow image of the wafer on a blue 

 ground. 



It has already been said that these images display themselves not only to the 

 open, but to the closed eye, and with the most surprising alternation of colors 

 and brightness, so that, after the observation of some definite object, these re- 

 peatedly change, in the secondary image, in a regular series, until, growing- 

 weaker and weaker, the image finally disappears. The following is an example 

 of this series of appearances, but it should be premised that by most eyes they 

 are only perfectly recognized after long practice and familiarity ; but then so 

 plainly and readily that the secondary images obtrude themselves among the 

 perceptions without particular attention, and often mingle inconveniently and 

 anuoyingly with the direct observations of sight. If we observe persistently a 

 bright-colored object — for instance, a bright candle flame through red glass — 

 and then closing the eye, direct our whole attention to the dark field of vision, 

 the secondary images of the red flame will present themselves in the following 

 succession : First, the original appearance subsists, like every impression of the 

 retina, for a very brief moment ; next, though so evanescent as to be almost 

 always overlooked, appears a bright green image of the flame on a dark ground; 

 this gives way to a bright red image of the flame, also on a dai-k ground, to 

 which succeeds a dark green image on a bright ground ; then again a bright red 

 on a dark ground ; again, a dark green on a bright ground ; and this alternation 

 proceeds until the whole phenomenon closes with a faint, dark green image on 

 a bright ground. Thus secondary images of like color and like relations of 

 brightness with the object are interchangeably mingled with those which appear 

 in the complementary colors of the object, and in which what was dark in the 

 object (the ground) is bright; and, inversely, what was bright (the flame) is 

 dark. Secondary images with such inverted relations of brightness are termed 

 negative ; those in which these relations are direct are termed positive. Inde- 

 scribably gorgeous is the play of colors of these images in the closed eye after 

 the contemplation of the sun itself. The positive image appears not white like 

 the siin, but assumes, one after the other, the most various and brilliant colors. 

 First, it appears of a bright blue with orange- colored border, then blue, violet, 

 deep red, with always differently tinted edges. Yet, however beautiful and 

 surprising these images of the sun, we should warn every one against a pro- 

 tracted observation of that luminary ; the experiment is extremely hazardous 

 for the eye ; the dazzling solar light over excites the retina and paralyzes the 

 vision ; the images once seen follow the observer with painful pertinacity. The 

 most striking attestation to the danger of such observations is found in the fact 

 that the inquirer who, with unwearied assiduity, first thoroughly investigated 

 these phenomena, soon grew blind, and has not yet fully recovered his sight. 

 To many of our readers he is not unknown ; under the name of " Mises " he 

 has endowed CTcrman literature with many highly poetical creations; and who- 

 ever, among his attractive writings, has met with the touching poem, " Der 

 Schwarze Vogel," (The Bird of Gloom,) will read it with double interest when 

 he knows that it sprung as a mournful effusion from his heart, when, in conse- 

 quence of his experiments, the veil of blindness was falling upon his eyes. We 

 renounce any further discussion of the details, as well as an enumeration of the 

 differently modified experiments relating to secondary images, and unfortunately 

 must, at the same time, renounce the purpose of explaining the nature of the 

 phenomenon itself, and the changes which it undergoes under different circum- 

 stances. There arc theories upon the subject, but none which are entirely satis- 

 factory. So much, indeed, we know, that these images are no dream images, 

 but secondary and necessary effects of the activity of the optic nerve and its ter- 



