4(l6 .THE SENSORIAL FUNCTIONS. 



in the appearance of an entire luminous circle, 

 from the rapid whirling round of a piece of lighted 

 charcoal ; for the part of the retina which receives 

 the brilliant image of the burning charcoal, re- 

 tains the impression with nearly the same intensity 

 during the entire revolution of the light, when the 

 same impression is renewed. For the same reason 

 a rocket, or a fiery meteor, shooting across the sky 

 in the night, appears to leave behind it a long- 

 luminous train. The exact time during which these 

 impressions continue, after the exciting cause has 

 been withdrawn, has been variously estimated by 

 different experimentalists, and is very much influ- 

 enced indeed, by the intensity of the impression.* 



* Many curious visual illusions may be traced to the operation 

 of this principle. One of the most remarkable is the curved ap- 

 pearance of the spokes of a carriage wheel rolling on the ground, 

 when viewed through the intervals between vertical parallel bars, 

 such as those of a palisade, or Venetian window-blind. On studying 

 the circumstances of this phenomenon, I found that it was the 

 necessary result of the traces left on the retina by the parts of each 

 spoke which became in succession visible through the apertures, 

 and assumed the curved appearances in question. A paper, in 

 which I gave an account of the details of these observations, and 

 of the theory by which I explained them, was presented to the 

 Royal Society, and published in the Philosophical Transactions, for 

 18"25, p. 131. About the year 1831, Mr. Faraday prosecuted the 

 subject with the usual success which attends all his philosophical 

 researches, and devised a great number of interesting experiments 

 on the appearances resulting from combinations of revolving wheels ; 

 the details of which are given in a paper contained in the first 

 volume of the Journal of the Royal Institution of Great Britain, p. 

 205. This again directed my attention to the subject, and led me 

 to the invention of the instrument which has since been introduced 

 into notice under the name of the Phantasmascope or Phenakisti- 

 cope. I constructed several of these at that period, (in the spring 

 of 1831) which I showed to many of my friends; but in conse- 

 quence of occupations and cares of a more serious kind, I did not 

 publish any account of this invention, which was reproduced on the 

 continent in the year 1833. 



