524 THE SENSORIAL FUNCTIONS. 



shooting across the sky in the night, appears to 

 leave behind it a long luminous train. The 

 exact time during which these impressions con- 

 tinue, after the exciting cause has been with- 

 drawn, has been variously estimated by different 

 experimentalists, and is very much influenced 

 indeed, by the intensity of the impression.* 



When the impressions are very vivid, another 

 phenomenon often takes place; namely, their 



* Many curious visual illusions may be traced to the ope- 

 ration of this principle. One of the most remarkable is the 

 curved appearance 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 appear- 

 ances in question. A paper, in which I gave an account of 

 the details of these observations, and of the theory by which I ex- 

 plained them, was presented to the Royal Society, and published 

 in the Philosophical Transactions, for 1825, p. 131. About 

 three years ago, 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 intro- 

 duced into notice under the name of the Phantasm ascope or 

 Phenakisticope. I constructed several of these at that period, 

 (in the spring of 1831) which I showed to many of my friends ; 

 but in consequence of occupations and cares of a more serious 

 kind, I did not publish any account of this invention, which was 

 last year reproduced on the continent. 



