JOURNAL 



THE ROYAL INSTITUTION 



GREAT BRITAIN. 



ON A PECULIAR CLASS OF OPTICAL DECEPTIONS. 



BY M. FARADAY, F.R.S. 

 Director of the Laboratory of the Royal Institution, &c. &c. 



PHE pre-eminent importance of the eye as an organ of 

 perception confers an interest upon the various modes in 

 which it performs its office, the circumstances which modify 

 its indications, 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 ex- 

 planation 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 shewn me moving with such velocity, that if 

 the eye were retained immovable, 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, junr. 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- 



VOL. I. FEB. 1831. P 



