peculiar Class of Optical Deceptions. 223 



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 

 appearance 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 Fig. 14 to represent a fixed circular brush, with long 

 hairs, and the little dots to be the sections of so many wires, 

 forming the arms of a frame which, when turned round, shall 

 carry the hairs of the brush forward a little, and then, letting 

 them go, allow them to return quickly to their first position. 

 If this frame be turned continually round, it would cause the 

 brush, when looked at from a distance, to appear as a revolving 

 toothed wheel, although in reality it had no circular motion. 

 Now, what is performed here by the wire-arms at the outer 

 extremity of the hairs, and the natural elasticity of the latter, 

 may, in the wheel animalcula, be effected at the roots of the 

 fibrillge by muscular power ; and in this or some similar way 

 the animal may have the power of urging the current necessary 

 to supply food, and, at the same time, producing the spectrum 

 of a continually revolving wheel, or even the more complicated 

 forms discovered by Leeuwenhoek (Fig. 15), without requiring 

 any powers beyond those which are within the understood laws 

 of Nature, and known to exist in the animal structure. 



Royal Institution, Dec. 10, 1830. 



Q2 



