306 On a Peculiar Class of Optical Deceptions. [1831. 



each side of the head ; they seem formed of deep teeth or short 

 radii, perhaps fourteen or fifteen in number ; the form of these 

 teeth is not sharp or well defined, but hazy at the edges ; the 

 interval between them is perhaps rather more than the width 

 of the teeth ; the teeth are not distinctly set on to a nave or 

 axis, but appear sometimes even to melt away or attenuate 

 at the part towards the centre, and sometimes appear as inde- 

 pendent portions, i. e. as much separated from the centre 

 part or supposed place of attachment as from the neighbouring 

 teeth. 



These parts are never seen as wheels, except in motion ; the 

 animal is sometimes seen without them, the parts which produce 

 the appearance being then either retracted and drawn inwards, 

 or disposed in other forms, for the animal is of a very change- 

 able nature. The motion of the wheels is continuous, as if 

 they were spinning constantly in one direction upon their axis ; 

 the velocity is such as to carry the teeth rapidly before the eye, 

 but is not enough to confound the impression of one tooth 

 with that of its neighbours, and therefore they may be distinctly 

 seen. Both wheels move usually in the same direction ; and 

 when the head of the animal is towards the observer, the 

 direction is generally the same as that of the hands of a clock. 

 Baker states, however, that he has seen them move in opposite 

 directions, and also has seen the motion first discontinued, and 

 then reversed, in the same wheel. The velocity is not always 

 the same, but varies with the efforts of the animal to catch its 

 food. Whatever the mechanism of the parts, the result is, that 

 currents are established 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 



