ON ACCIDENTAL OR SUBJECTIVE COLORS. 241 



at the end of some instants, a light of yellowish white tint. Almost at the same 

 time this light will he scparatoclas into small lozenges, which will distribute them- 

 selves regularly upon a fasciculus of right lines converging towards the same 

 centre, and which do not appear to deviate by more than 45° on eacli side from 

 the perpendicular to the right line which passes by the centres of the two eyes. 

 The fasciculus of right lines, shows itself but for an instant and appears to be 

 transformed into hyperbolas, all having for a common axis the perptndicular 

 just spoken of, and common foci wherein are seated two uniform and reddish 

 spots; these foci then swerve aside and the groundwork of the brilliant tableau 

 assumes an undulatory appearance. When the pressure has ceased or is relaxed 

 there is no longer anything perceived except a black spot siuTOunded with yel- 

 lowish light and covered with red and yellow filaments, which vibrate with great 

 rapidity. ^\ hen the eyes are kept covered, this spot and the circle which sur- 

 rounds it take in the end a uniform yellowish tint, which long persists and 

 vanishes gradually. This phenomenon is rarely seen with all the circumstances 

 we have described, because some practice is necessary to produce and to observe 

 it, and the pressure on the eyes, which is quite painful, must be sufficiently 

 strong. 



2. Electricity. — The galvanic commotion produces not only a species of flash 

 before the eyes, but the sensation of different colors, according to the different 

 application of the electric poles. When the zinc or positive pole is placed in 

 the mouth, and the copper or negative pole in the middle of the forehead, the eye 

 receives the impression of a spot of violet color; if the poles be changed the 

 spot becomes yellow. 



3. Imagination. — This, in certain persons, acts in such a manner upon the 

 organ of vision that if they represent to themselves any object, that object is 

 instantly displayed before their eyes with its natural color and form. Goethe 

 assures us that when he closed his eyes and thought of any flower it immedi- 

 ately presented itself before him. Whenever Newton, after the experiment of 

 which we have spoken, thought of the sun, that orb appeared to him at the 

 instant, even in the midst of the most profound darkness. After the lapse of 

 years he would have been able, had he chosen to encounter the risk, to repro- 

 duce at will, by the power of his imagination, the sensations which he had be- 

 fore experienced. 



4. Internal causes. — Sensations of color are also evoked by congestion of the 

 blood, lympliatic irritation of the eye, inflammation, &c. In these particular 

 cases of indisposition the pressure of the blood vessels on the retina produces 

 floating masses of light visible in the dark ; first of pale blue, then green, then 

 yellow, and sometimes even of red. All these colors are occasionally seen on 

 the borders of the luminous mass 



Undulations excited in the retina by the action of luminous points and lines. — 

 The following observations we owe to Sir D. Brewster: If we look through a 

 narrow opening of about a demi-millimetre towards a bright portion of the sky 

 or the flame of a candle, the luminous background will be seen covered with a 

 great number of broken parallel lines, alternately lustrous and obscure. These 

 lines arc always parallel to the narrow cleft, and naturally change their place 

 while the clclt is turned circularly before the eye. Through a certain number 

 of parallel clefts, as through the teeth of a comb, the parallel broken lines are 

 seen more distinctly, and, if an oblique motion be given to the comb in the di- 

 rection of its teeth, the broken lines hecome still more distinct, although not so 

 straight as at first, and we see new black lines placed in different directions, a& 

 if they were detached portions of a great number of dark ramifications. All 

 these phenomena are better seen by employing homogeneous light. If we take 

 two systems of narrow clefts and cross them under different angles, we shall 

 see two systems of broken lines crossing each other under the same angles ; and 

 if, when the lines of the two systems are parallel, we give to one of th(;m a 

 IG s GO 



