LAW OP VISIBLE DIEECTIOtf. 



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intuitive knowledge of the position of any remote stimulus is 

 realised : we only infer its position from the particular consciousness 

 of which the mind is the subject. Suppose, for instance, that the 

 eyes are directed to a small luminous object at a little distance. A 

 remote stimulus is not intuitively known even to exist. Dr. Reid, 

 indeed, the founder of the Scottish School of Philosophy, taught that 

 distant objects are immediately perceived : but this doctrine will no 

 longer find a single intelligent defender. As Sir "William Hamilton 

 has pointed out, Reid here fell into a fatal inconsistency. Those 

 metaphysicians who believe that material objects have an existence 

 at all, apart from the mind, are now unanimous in admitting that 

 distant material objects, like the luminous point referred to, are not 

 immediately perceived ; and I presume that Sir David Brewster 

 would himself subscribe to this view, when formally presented to him. 

 This leads at once to the result, that the visible position of a distant 

 object is indefinite ; for, the estimate which we form of the position, 

 or of any of the relations, of a thing not immediately known, is liable 

 to variation. Different persons, and even the same person at different 

 times, may form extremely different estimates of the position of a 

 point. But if visible direction be thus indefinite, it cannot be capable 

 of being expressed by a definite law, either that of Sir David Brew- 

 ster, or any other. 



It might be thought, indeed, from a superficial view of the 

 subject, that, in opposition to what has been said, impressions made 

 upon the retina are determinately referred to particular directions. 

 Is not every one, it may be asked, familiar with the fact that objects 

 often appear where the observer knows them not to be, and where, 

 nevertheless, he cannot help fancying them to be ? An object is 

 known to be at A. The sense of touch assures us that it is so. Yet 

 it appears to be at B. We are obliged, in spite of ourselves, to refer 

 the visual impression to a stimulus in the position B, though our 

 reason is satisfied that such reference is erroneous. No t ffort, as 

 Sir David Brewster says, in describing a case of the kind, is suffici- 

 ent " to dispel the illusion." Does not this prove that impressions 

 made upon the retina are instinctively referred to particular definite 

 directions ? I answer : no. Take the simplest of all examples. To 

 an observer looking at an object reflected from a plane mirror, the 

 image appears (to speak popularly) behind the mirror. Now here 

 undoubtedly a determinate effect is produced ; an effect which no 

 knowledge possessed by the observer, nor any effort of his will, can 

 modify. But what is this determinate effect ? It is the image formed ; 



