WHY WE DO NOT SEE DOUBLE. 



imagine that the mind can receive two distinct perceptions of the 

 object, for it is not possible that the two perceptions could differ 

 from each other in any respect, except in some of those just men- 

 tioned. Let us suppose the two eyes to look at the moon, and that 

 such object impresses them with an image of precisely the same 

 apparent form and magnitude, of precisely the same colours 

 and lineaments, of precisely the same intensity of illumination, 

 and in precisely the same direction. Now, the impressions 

 conveyed to the mind by each of the eyes corresponding in all 

 these respects, the object must be perceived in virtue of both 

 impressions precisely in the same manner, that is to say, it must 

 be seen in precisely the same direction, of precisely the same 

 magnitude, of precisely the same form, with precisely the 

 same lineaments of light and shade, and with precisely the 

 same brightness or intensity of illumination. It is therefore, 

 in such a case, clearly impossible to have a double perception of 

 the object. 



It will be observed, that the same reasoning exactly will be 

 applicable to the sense of hearing. If the same string or the 

 same pipe affect the tympanum of each ear in precisely the same 

 manner, so as to produce a perception of a sound of the same pitch, 

 the same loudness, and the same quality, it is impossible to con- 

 ceive that two different perceptions can be produced by the two 

 ears, for there is no respect in which it is possible for two such 

 perceptions to differ, inasmuch as by the very supposition they 

 agree in all the qualities which belong to sound. 



But, if we could conceive by any organic derangement that the 

 same musical string would produce in one ear the note ut, and 

 produce in the other ear the note so/, then the same effect would 

 be produced as if these two sounds had been simultaneously heard 

 by the two ears properly organised, and we should have a sense of 

 harmony of the fifth. 



In like manner, if the two eyes, by any defect of organisa- 

 tion, produced different pictures on the retina, we should then 

 have two perceptions of the same object having a corresponding 

 difference. 



It has been already shown, that the apparent visual magnitude 

 of an object, and also that its apparent brilliancy, depend on its 

 distance from the eye. 



Now, assuming, as we shall do, unless the contrary be expressed, 

 that the two eyes are similarly constituted, it will follow, that an 

 object whose distance from the two eyes is equal will be seen 

 under the same visual angle, and will therefore have the same 

 apparent magnitude ; it will also have the same colour and 

 intensity of illumination, and, in fine, if the distance between the 



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