ON ACCIDENTAL OR SUBJECTIVE COLORS. 243 



narrow fissure is before the eyes, and either the fissure or the head is inclined, 

 the stripes change their place and arrangement. From these facts it seems evi- 

 dently to result that the stripes in question are formed in the eye. Their origin 

 may be satisfactorily explained by admitting that there exists in the organ a 

 certain number of obscure points of very small diameter ; for each luminous 

 point of the fissure will project on the retina a shadow of that point, and the 

 series of shadows projected by the different points? of the fissure form a dark line 

 which will be parallel to it; this hypothesis meets all the requirements of the 

 phenomenon. The accuracy of this explanation is confirmed, moreover, by de- 

 cisive experiments. Let a plate of glass, on which has been made with India 

 ink a very small black point, be placed between the eye and a narrow fissure, 

 and a dark stripe will be seen parallel to the fissure If the sky be observed 

 through a narrow fissure whose length we can at will diminish by means of a 

 movable plate whose edge is perpendicular to its direction, when the orifice has 

 a length which differs little from the width, we shall perceive a circular field 

 strewn with dark points, always disposed in the same manner in regard to the 

 eye ; and by the elongation of the fissure each dark point produces a stripe. 

 As to the nature of the dark points of the eye, M. Peclet thinks that they pro- 

 ceed fiom the mammillated structure of the transparent cornea, or from the en- 

 velope of the aqueous humor ; for each little mammilla will act as a lens of short 

 focus; the light which traverses them will be dispersed in a very open cone; 

 and each of them will cast upon the retina a shadow like an opaque body. 



We must seek in the stripes of M. Peclet the explanation of an illusion by 

 which M. Zantedeschi has allowed himself to be deceived. This Italian physi- 

 cist has announced to the learned world, as a discovery worthy of attention and 

 one which had escaped the penetrating eye of Fraunhofer, the presence in the 

 solar spectrum of bands not transversal, but longitudinal or parallel to the 

 lengthwise dimensions of the spectrum. Had they existed as an integral part 

 of the spectrum, equally with the bands of Fraunhofer, these new dark bands 

 would have been wholly inexplicable, or rather uninteUigible. They are fortu- 

 nately but an extrinsic peculiarity ; they doubtless proceed either from the par- 

 ticles of dust inseparable from the edges of the fissure, or perhaps from the dark 

 globules of the eye. 



There remains to be noticed a singular fact observed by M. Libri. If a black 

 and vertical line of little length be traced on a white wall, and we station our- 

 selves at a little distance from this wall, a distance which will vary, for the same 

 observer, with the width of the line, the quantity of reflected light, &c. ; if, 

 further, after having closed one eye, a very slender thread be placed near the 

 other, so that in looking at the wall and the line there traced, the image of the 

 thread shall seem to cut this line obliquely, the thread will appear, at the point 

 of intersection, to be divided, so to say, into two parts, one of which will be 

 elevated parallel to its first direction, and the other will be lowered. These 

 parts will leave between their corresponding extremities a small void space, which 

 will be precisely the place that the true image ought to occupy. It should be 

 observed that the superior part will always appear lowered, and the inferior 

 part, on the contrary, will appear above its real height. If the obliquity of the 

 thread be diminished, and little by little its projection be brought to cut perpen- 

 dicularly the line traced on the wall, the two extremities of the images will be 

 seen gradually to approach each other, and then to form a single thread ; and 

 finally the same appearances will present themselves in an inverse direction, 

 when by continuing to make the thread turn, we render it again oblique in a 

 position opposite to the first. 



On ■processes to render visible the ramifications of the retina. — The following 

 experiment has facilitated, in a physiological point of view, the explanation of 

 a certain number of facts ; we owe it to Professor Purkinje, of Breslau : If a 

 lighted candle be held before the eye, at the distance of about a foot, in a dii-ec- 



