ON SENSORIAL VISION. 4OI 



organs. It is to this latter division of the subject that I 

 shall chiefly address myself, while taking the opportunity 

 thus kindly afforded me of putting on record certain 

 visual phenomena which I have from time to time noticed, 

 belonging to that obscure class of impressions which 

 may be termed Sensorial Vision by which I mean visual 

 sensations or impressions bearing a certain considerable 

 resemblance to those of natural or I'ethial vision, but 

 which difter from these in the very marked particular 

 of arising when the eyes are closed and in complete 

 darkness. 



(2.) Few persons, I suppose, are ignorant, as a matter 

 of personal experience, of the sort of appearances known 

 by the name of Ocular Spectra, which are produced by 

 the impression of a strong light on the retina of the eye, 

 and which continue to force themselves on the attention, 

 sometimes in a very pertinacious and disagreeable way 

 for some time afterwards, when the eyes are closed. In 

 one lamentable instance, that of an eminent Belgian 

 Philosopher, they have caused actual loss of sight ; and 

 in that of Sir Isaac Newton, their obstinate recurrence is 

 said to have deprived him of sleep for several days and 

 nights successively, and to have driven him to the verge 

 of distraction. These are cases when the stimulus of 

 light has been pushed to the extreme ; but when mode- 

 rate and regulated, these spectra admit of being studied: 

 and the laws of their production^ the singular and beau- 

 tiful phases they pass through their periodical extinc- 

 tion and renewal (which extend over a very considerable 

 interval of time from their first production), the orderly 



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