peculiar Class of Optical Deceptions. 221 



mechanism of the parts, the result is, that currents are esta- 

 blished in the water towards the head of the animal, which 

 currents pass off outward from the edges of the apparent 

 wheels ; and little particles floating in the water may be seen 

 to pass towards the head, and be suddenly thrown off at the 

 edges of the wheels with considerable force. 



So striking are the appearances of these animalcula, that 

 men of much practice in microscopical observation are at this 

 day convinced they do possess wheels, which actually revolve 

 continuously in one direction. The struggle in Mr. Baker's 

 mind between the evidence of his senses and his judgment, 

 illustrates this point in so lively a manner, that I may be 

 excused quoting his account of it : ' As I call these parts 

 wheels, I also term the motion of them a rotation, because it 

 has exactly the appearance of being such. But some gentle- 

 men 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 diffi- 

 culty 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 supposed 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 communication 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 pro- 

 truded they never fail to shew all the visible marks imaginable 

 of a regular turning round ; which 1 think no less difficult to 

 account for, if they do not really do so. Nay, in some posi- 

 tions 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 simi- 

 litude. However, as a man can move his arms or his legs 

 VOL. I. FEB. 1831. Q 



