1831.] On a Peculiar Class of Optical Deceptions. 307 



appearance of being such. But some gentlemen have imagined 

 there may be a deception in the case, and that they do not 

 really turn round, though indeed they seem to do so. The 

 doubt of these gentlemen arises from the difficulty they find in 

 conceiving how or in what manner a wheel or any other form, 

 as part of a living animal, can possibly turn upon an axis sup- 

 posed to be another part of the same living animal, since the 

 wheel must be a part absolutely distinct and separate from the 

 axis whereon it turns ; and then say they, how can this living 

 wheel be nourished, as there cannot be any vessels of communi- 

 cation between that and the part it goes round upon, and which 

 it must be separate and distinct from ? To this I can only 

 answer, that place the object in whatever light or manner you 

 please, when the wheels are fully protruded they never fail to 

 show all the visible marks imaginable of a regular turning 

 round ; which I think no less difficult to account for, if they do 

 not really do so. Nay, in some positions you may, with your 

 eye, follow the same cogs or teeth whilst they seem to make a 

 complete revolution ; for the other parts of the insect being very 

 transparent, they are easily distinguished through it. As for 

 the machinery, I shall only say, that no true judgment can be 

 formed of the structure and parts of minute insects by imaginary 

 comparisons between them and larger animals, to which they 

 bear not the least similitude. However, as a man can move his 

 arms or his legs circularly as long and as often as he pleases by 

 the articulation of a ball and socket, may there not possibly be 

 some sort of articulation in this creature whereby its wheels or 

 funnels are enabled to turn themselves quite round ? 



" It is certain all appearances are so much on this side of the 

 question, that I never met with any who did not, on seeing it, 

 call it a rotation ; though, from a difficulty concerning how it 

 can be effected, some have imagined they might be deceived. 

 M. Leeuwenhoek also declared them to be wheels that turn 

 round (vide Phil. Trans., No. 295). But I shall contend with 

 nobody about this matter : it is very easy for me, I know, to be 

 mistaken, and so far possible for others to be so too, that 1 am 

 persuaded some have mistaken the animal itself, which perhaps 

 they never saw ; whilst, instead thereof, they have been exa- 

 mining one or other of the several water-animalcules that are 

 furnished with an apparatus commonly called wheels, though 



