THE EYE. 



question, is why the same question has not been similarly pro- 

 posed with reference to the sense of hearing. Why has it not 

 been asked why we do not hear double ? why each individual 

 sound produced by a bell or a string is not heard as two distinct 

 sounds, since it must impress independently and separately tho 

 two organs of hearing ? 



It cannot be denied, that, whatever reason there be for demand- 

 ing a solution of the question, why we do not see double 't is 

 equally applicable to the solution of the analogous question, why 

 we do not hear double ? Like many disputed questions, this will 

 be stripped of much of its difficulty and obscurity by a strict 

 attention to the meaning of the terms used in the question, and in 

 the discussion consequent upon it. If by seeing double it be 

 meant that the two eyes receive separate and independent impres- 

 sions from each external object, then it is true that we see double. 

 But if it be meant that the mind receives two distinct and inde- 

 pendent impressions of the same external object, then a qualified 

 answer only can be given. 



If the two eyes convey to the mind precisely the same impres- 

 sion of the same external object, differing in no respect whatever, 

 then they will produce in the mind precisely the same perception 

 of the object; and as it is impossible to imagine two perceptions 

 to exist in the mind of the same external object which are 

 precisely the same in all respects, it would involve a contradic- 

 tion in terms to suppose that, in such case, we perceive the object 

 double. 



If to perceive the object double mean anything, it means that 

 the mind has two perceptions of the same object, distinct and dif- 

 ferent from each other in some respect. Now, if this distinctness 

 or difference exist in the mind, a corresponding distinctness and 

 difference must exist in the impression produced of the external 

 object on the organs. It will presently appear, that cases do occur 

 in which the organs are, in fact, differently impressed by the same 

 external object; and it will also appear, that in such cases pre- 

 cisely we do see double, meaning by these terms, that we have two 

 perceptions of the same object, as distinct from each other as t.re 

 our perceptions of two different objects. 



To render this point more clear, let us consider in what respects 

 it is possible for the impressions made upon the two eyes by the 

 same object to differ from each other. 



A visible object impresses the eye with a sense of a certain 

 apparent form, of a certain apparent magnitude, of certain colours, 

 of a certain intensity of illumination, and of a certain visible 

 direction. Now, if the impression produced by the same object 

 upon the two eyes agree in all these respects, it is impossible to 

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