1831.] On a Peculiar Class of Optical Deceptions. 291 



then again heated to ensure the dissipation of all volatile 

 matter. 



After this is done, the platinum should be rubbed to powder 

 by the clean finger on clean paper (83), heated slightly a third 

 time, and then preserved in a clean and well-stopped bottle. 



On a Peculiar Class of Optical Deceptions *. 



THE pre-eminent importance of the eye as an organ of per- 

 ception confers an interest upon the various modes in which 

 it performs its office, the circumstances which modify its indi- 

 cations, and the deceptions to which it is liable, far beyond 

 what they otherwise would possess. The following account of 

 a peculiar ocular deception, which, in a greater or smaller 

 degree, is not uncommon, and which, if looked for, may be 

 observed with the utmost facility, may therefore prove worthy 

 of attention ; and I am the more inclined to hope so, because 

 in some points it associates with an account and explanation of 

 an ocular deception given by Dr. Roget in the ' Philosophical 

 Transactions' for 1825, page 121. 



The following are some cases of the appearance in question. 

 Being at the magnificent lead mills of Messrs. Maltby, two 

 cog-wheels were shown me moving with such velocity, that if 

 the eye were retained immoveable, no distinct appearance of 

 the cogs in either could be observed ; but, upon standing in 

 such a position that one wheel appeared behind the other, 

 there was immediately the distinct though shadowy resem- 

 blance of cogs moving slowly in one direction. 



Mr. Brunei, jun. described to me two small similar wheels 

 at the Thames Tunnel: an endless rope, which passed over 

 and was carried by one of them, immediately returned and 

 passed in the opposite direction over the other, and conse- 

 quently moved the two wheels in opposite directions with great 



* Quarterly Journal of Science, 1831, vol. i. p. 205. 



I take the opportunity here of pointing out that, three years prior to my 

 paper, Professor Plateau had published an account of the chief fact, in the 

 fourth volume of the ' Correspondence Mathematique et Physique ' of M. 

 Quetelet, p. 393. I was of course unaware of the circumstance. Further 

 observations by M. Plateau will be found in the ' Annales de Chimie/ 1831, 

 xlviii. p. 281. 



u2 



