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On Single and Correct Vision, by means of Double and Inverted 

 Images on the Retincp. By W. P. Alison, M.D. F.R.S.E. 

 Professor of the Institutes of Medicine in the University of 

 Edinburgh. 



(Read llth April 1836.) 



In entering on a question which may be said to occupy a por- 

 tion of the debateable land between Physiology and Metaphy- 

 sics, it seems, in the first place, necessary to state with precision 

 the nature of the difficulty, which has long been felt on this sub- 

 ject, and endeavour to determine the degree to which it is rea- 

 sonable to expect, that this difficulty may be removed ; and on 

 these points there is such a discrepancy of opinion, even among 

 the latest and most esteemed authors, as obviously to make far- 

 ther inquiry desirable. 



No one can be more thoroughly convinced than I am, of the 

 utter futility and absurdity of all attempts " to shoot the gulf 

 which separates the sensible world from the sentient soul." In 

 all our inquiries in the Physiology of the Nervous System, as 

 connected with mental acts, we must keep in mind, that the end 

 of these inquiries can only be, to determine the physical condi- 

 tions under which the different mental phenomena take place ; 

 and those under which, when they have taken place, they affect 

 the different organs of the body. The question, how it comes 

 about, that when those conditions are fulfilled, these results fol- 

 low, must be held, in every case, to be beyond our powers. 



But as it is clearly in our power to ascertain the general condi- 

 tions under which any mental phenomena are connected with a 

 living body, so it may also be in our power to ascertain the special 

 conditions under which any particular idea, or other mental act, 

 takes place ; and particularly, to determine the exact sensations 

 with which any particular notion formed in the mind is naturally 



