216 ON ACCIDENTAL OR SUBJECTIVE COLOEs'. 



circumstances, by the flame of a candle subjected to this mode of observation. 

 We know that such a flame seems, from time to time, to undergo a rapid oscil- 

 latory movement in the direction of its length ; its summit appears to rise and 

 sink alternately. If, at the moment of such agitation, this flame be observed 

 by means of the disk in question, the upper part will be seen to bs divided into 

 several distinct portions, situated some above the others, and separated by dark 

 intervals. From this it must be inferred that the apparent oscillatory movement 

 of the flame is occasioned by the detachment from its summit of a succession of 

 partial and separate flames which rise rapidly, and are extinguished one after 

 the other. 



Thaumatrope. — The phenomenon which we have been considering is the 

 principle of many curious illusions. Every one is acquainted with the little in- 

 strument called a thaumatrope, which consists of a circular piece of pasteboard 

 made to move rapidly around one of its diameters, and which bears on each of 

 its faces figures drawn in such a way that the combination of the two impres- 

 sions they produce on the retina forms a third and regular picture. 



Anorthoscope. — M. Plateau has made use of the persistence of impressions to 

 produce a new sort of anamorphosis by causing two disks to turn rapidly, one 

 behind the other, with velocities relatively determinate, the disk in the rear 

 bearing a distorted figure brightly illuminated, and that in front being perforated 

 with a narrow opening. He has shown that from the most deformed figures 

 may thus be produced regular images which seem perfectly motionless. The 

 regular figures thus generated result from the successive apparent intersections 

 of the opening with difierent parts o.f the deformed figure ; intersections which 

 the persistence of impressions causes to appear simultaneous. This application 

 has given rise to a curious apparatus, known by the name of anorthoscope. 



Experiments of Mr. Faraday.— Mr. Faraday has published a note on certain 

 illusions resulting principally from the regular appearances presented by two 

 wheels of the same size and with the same number of radii, placed one behind 

 the other, and turning rapidly on a common axis in planes very close to one 

 another, with equal velocities and in contrary directions. The eye, placed in 

 front of such a system in the direction of the axis, will see the appearance of a 

 wheel perfectly motionless, and whose radii will be double in number to those 

 of each of the two wheels in motion. 



Another experiment by Mr. Faraday consists in causing to turn, in front of a 

 mirror and four to five metres distant from it, a dentatcd wheel of pasteboard, 

 and observing in the glass the image of this wheel through the seemingly gauzy 

 medium produced by the movement of the teeth and their intervals, the eye 

 being placed very near the wheel. The image then appears completely mo- 

 tionless and in its real form, as if the movement of the wheel had ceased. Mr. 

 Faraday has further shown that, if the part of the wheel comprised between the 

 teeth and the centre be divided into sectors colored and suitably arranged, the 

 colors, which become confused when the revolving wheel is looked at directly, 

 are immediately separated if its image in the mirror be observed in the manner 

 above indicated. It would be easy to deduce from the principles which we 

 have already estabhshed the reason of the effects just described. In the same 

 manner may be explained the following experiment, due also to Mr. Faraday: 

 If a new range of openings be cut in the wheel between the teeth and the cen- 

 tre, the number of which differs a little from that of the teeth, and this wheel 

 be then subjected to the experiment of the mirror, on looking through one or 

 another range of openings, the image of 'that through which we look appears to 

 undergo a slow movement of rotation. Several rows of openings may be thus 

 pierced so as to produce the appearance of movements more or less rapid and of 

 different directions. 



Phantascope, phenakistoscope. — The experiments which we have been re- 

 counting have led M. Plateau to realize a new species of illusions, which con- 



