220 Mr. Faraday on a 



The curious appearance exhibited by the wheel animalcule 

 has such a resemblance to some of those described in this 

 paper, that they inevitably associate in the mind of a person 

 who has witnessed both effects. This little insect has been well 

 described by Mr. Baker * and others, and can only be viewed 

 distinctly under a high magnifying power ; it then presents an 

 elongated sack-like form (Fig. 17.), either attached by the 

 posterior part to the side of the vessel containing the water in 

 which it exists, or else floating in the fluid. When the effect 

 in question is observable, there is seen the appearance of two 

 wheels, one on each side of the head ; they seem formed of 

 deep teeth or short radii, perhaps fourteen or fifteen in num- 

 ber ; 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 toward the centre, and some- 

 times appear, as independent 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 pro- 

 duce the appearance being then either retracted and drawn 

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

 changeable nature. The motion of the wheels is conti- 

 nuous, 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 there- 

 fore 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 



* Baker on the Microscope, vol. ii. p. 266 ; see also Leemvenhoek, Phil. Trans., 

 Nos. 283, 295, 337 ; and Adams on the Microscope, p. 548. 



