On a Peculiar Class of Optical Deceptions. [1831. 



they turn not round, but excite a current by the mere vibration 

 offibrillae about their edges." 



Notwithstanding the evidence adduced by Mr. Baker, which, 

 as I have said, is admitted by some at the present day, it must 

 be evident, from a consideration of the nature of muscular force, 

 and the condition of continuity under which all animals exist, 

 that the rotation cannot really occur. The appearances are 

 altogether so like some of those exhibited in the experiments 

 already described, that I feel no doubt the wheels must be con- 

 sidered not as having any real existence, but merely as spectra, 

 produced by parts too minute, or else having too great a velocity 

 when in use by the animal to be themselves recognized. It is 

 not meant that they are produced by toothed or radiated wheels; 

 for that supposition would take for granted what has already 

 been considered as impossible continual revolution of one part 

 of an animal whilst another part is fixed ; but arrangements 

 may be conceived, which are perfectly consistent with the usual 

 animal organization, and yet competent to produce all the effects 

 and appearances observed. Thus, if that part of the head of 

 the animal were surrounded by fibrillae, endowed each with 

 muscular power, and projecting on all sides, so as to form a kind 

 of wheel ; and if these fibrils were successively moved in a 

 tangental direction rapidly the one way, and more slowly back 

 again, it is evident that currents would be formed in the fluid, 

 of the kind apparently required to bring food to the mouth of 

 the animal ; and it is also evident, that if the fibrils, either alone 

 or grouped many together, had any power of affecting the sight, 

 so as to be visible, they would be less visible at the part through 

 which they were rapidly moving, than that through which they 

 were slowly returning ; and at that place, therefore, an interval 

 would appear, which would seem to travel round the wheel, in 

 consequence of the successive action of the fibrils. But if, 

 instead of the whole group of fibrils acting in succession as 

 one series, they were to be divided by the will or powers of 

 the animal into fifteen or sixteen groups, the action being in 

 every other respect the same, then there would be the appear- 

 ance of fifteen or sixteen dark spaces, and as many light ones 

 disposed as a wheel ; and these would continue to travel round 

 in one direction, so long as the animal continued the alternate 

 action of the fibrils. This may be illustrated by supposing 



