353 THE SENSORIAL FUNCTIONS. 



time after the vibrations of the external medium have ceased; 

 as is shown by the sensation of a musical note being the re- 

 sult of the rco-ular succession of aerial undulations, when the 

 impression made by each continues during the whole inter- 

 val between two consecutive vibrations. The impulses of 

 lio-ht on the retina are unquestionably consecutive, like those 

 of sound, but being repeated at still shorter intervals, give 

 rise to a continuous impression. A familiar instance of the 

 same principle occurs in the appearance of an entire lumi- 

 nous circle, from the rapid whirling round of a piece of 

 liglUed charcoal; for the part of the retina which receives 

 the brilliant image of the burning charcoal, retains the im- 

 pression with nearly the same intensity during the entire 

 revolution of the light, when the same impression is re- 

 newed. 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 withdrawn, has been variously estimated by different experi- 

 mentalists, and is very much influenced, indeed, by the in- 

 tensity of the impression.* 



* Many curious visual illusions may be traced to the operation 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 bare, such as those of a palisade, or Vene- 

 tian window-blind. On studying the circumstances of this phenomenon, t 

 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 Koyal Society, and published in the 

 Philosophical Transactions, for 1825, p. 131. About three years ago, Mr. 

 Faraday prosecuted the subject with tlie usual success which attends all his 

 philosophical researches, and devised a great number of interesting experi- 

 ments on the appearances resulting from combinations of revolving wlipels; 

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

 Journal of tlie Koyal Institution of Great Britain, p. 205. Tliis again direct- 

 ed 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 Phantas- 



