ON ACCIDENTAL OR SUBJECTIVE COLORS. 215 



Other j)rocessvs for manifesting the continuity or discontinuity of various phe- 

 nomena. — Savart, in a particular case, made use of a process which in like manner 

 enabled him to rectify false appearances resulting from rapid movement, and to 

 determine the real figure of a liquid vein flowing from a circular orifice pierced 

 in a thin plate. His apparatus consists essentially of a large riband, whos^ sur- 

 face is divided by transverse bands alternately white and black, and to which a 

 rapid movement may be given in the direction of its length. The liquid being 

 supposed to flow vertically, the movable riband is placed behind the vein, and 

 in a position parallel to it. If a movement sufficiently rapid be then communi- 

 cated to the apparatus, the eye, placed before the system, can distinguish the 

 several peculiarities presented hy the vein ; such as the annular dilatations which 

 are propagated along the portions of the vein contiguous to the orifice ; a portion 

 which, without the aid of this instrument, appears as smooth as a stem of crystal. 

 The effect produced by this apparatus is analogous to those produced by two 

 wheels turning with rapidity, one behind the other, in planes parallel and ap- 

 proximated, and with unequal velocities of speed or direction. 



M. Matteucci has remarked quite recently that, in order to confirm the inge- 

 nious observations of Savart, on the constitution of the liquid vein, it is only requi- 

 site to illuminate the vein with a large electric spark, or rather a series of sparks. 

 On examining in this manner the fluid vein, obtained as Savart prescribes, it will 

 be easily seen that the part which, to the eye, appears continuous, is in reality 

 composed of drops having exactly the form assigned to them by the illustrious 

 physicist, some being elongated, others flattened, others almost spherical. 



M. Plateau has proposed a general process different from that of Mr. Wheat- 

 stone, to show under their real form the objects to which their rapid movement 

 gives a deceptive appearance. His apparatus consists of a disk of blackened 

 pasteboard, of about twenty-five centimetres in diameter, movable around an axis 

 like a wheel, and perforated towards its circumference with some twenty narrow 

 slits in the direction of the radii. These openings should be about two millime- 

 tres in breadth, and two centimetres in length, and situated at equal distances 

 from one another. To make use of this instrument, it is necessary to give to the 

 disk a movement of rotation sufficiently rapid, then closing one eye, to observe 

 with the other, through the transparent circular band which results from the revo- 

 lution of the slits, the movable object of which we wish to ascertain the true form ; 

 the eye, at the same time, must be held as near as possible to the openings, and 

 the observer be at a certain distance from the object. 



Let us suppose, in the first place, that the object has a periodical movement; 

 that is to say, that it passes successively through the same positions ; that, for 

 example, it is a chord in vibration, a burning coal moved circularly, &c. ; or 

 that such objects as the radii or teeth of a wheel, &c., occupy in succession the 

 same place ; then, if the velocity of our disk be such that whenever an opening 

 passes before the eye the same object or similar objects recur in the same posi- 

 tion, there will obviously be formed on the retina a succession of identical im- 

 pressions, which their persistence will connect with one another ; hence will re- 

 sult the continuous appearance of an object or series of immovable objects, 

 having the real form, or very nearly such, of the objects we are observing. 

 Thus, for instance, a wheel which turns so rapidly that its radii or teeth appear 

 confounded, would seem perfectly motionless through our disk, supposing tlic 

 latter to have a suitable velocity. Even if the movement of the object do not 

 present a regular succession, as in the case of one of those luminous meteors 

 whose brilliant trace is probably due to the persistence of the impression, the 

 instrument will still be useful. In that case the object cannot be isolated, as 

 regards the eye, in a succession of identical positions ; but the images which it 

 produces on the retina, in the different positions which correspond to the pas- 

 sage of the successive openings, will persist sufficiently long to enable us to 

 juds^e in general of its real form. 



In illustration, we will refer to the effect which is presented, under certain 



