1831.] On a Peculiar Class of Optical Deceptions. 303 



fig. 14 to represent a fixed circular brush, with long hairs, and 

 the little dots to be the sections of so many wires, forming the 

 arms of a frame which, when turned round, shall carry the 

 hairs of the brush forward a little, and then, letting them go, 

 allow them to return quickly to their first position. If this 

 frame be turned continually round, it would cause the brush, 

 when looked at from a distance, to appear as a revolving toothed 

 wheel, although in reality it had no circular motion. Now, 

 what is performed here by the wire- arms at the outer extremity 

 of the hairs, and the natural elasticity of the latter, may, in the 

 wheel animalcula, be effected at the roots of the fibrillse by 

 muscular power ; and in this or some similar way the animal 

 may have the power of urging the current necessary to supply 

 food, and, at the same time, producing the spectrum of a con- 

 tinually revolving wheel, or even the more complicated forms 

 discovered by Leeuwenhoek (fig. 15), without requiring any 

 powers beyond those which are within the understood laws of 

 Nature, and known to exist in the animal structure *. 

 Royal Institution, Dec. 10, 1830. 



[In Mr. Whitock's carpet and fringe-manufactory at Edin- 

 burgh, they were covering a cord with silk. The cord was of 

 two strands, differently coloured, slightly twisted, and was 

 turning rapidly round on its axis. In many places it looked 

 like a party-coloured cord perfectly still. This was from the 

 continual recurrence of portions different from their neighbours, 

 in the same place ; they were not visible all the way round, but 

 only above or below, or in some particular part of their revolu- 

 tion. July 1833. M. F.] 



ADDITIONAL NoTEf. 



In consequence of the necessity I was under of sending the 

 paper (page 291) referred to in the above* Proceedings ' to press 

 by a certain time, I was unable to pursue many of the beautiful 

 combinations of form, colour, and appearance to which the 

 experiments led, especially as they promised only amusement, 

 and little more of instruction than the paper itself contained ; 



* See in relation to this subject, Homer, on the Dsedaleum, Phil. Mag. 

 1843, iv. 36. 



t Quarterly Journal of Science, 1831, vol. i. 334. 



