276 ON SIR DAVID BREWSTER'S SUPPOSED LAW, ETC. 



and (as was previously pointed out) an image is a subjective phenome- 

 non —not, perhaps, purely so, but subjective at least in this sense, that 

 it has no existence in space, apart from the mind. In granting, however, 

 that the observer cannot by any effort modify the image, or subjective 

 phenomenon, connected with a particular impression made upon the 

 retina— which is just granting, in other words, that he cannot make 

 his perception any thing else than it is — we by no means grant that 

 experience does not enable him to modify his reference of the visual 

 impression to a remote stimulus. A child, or a savage, who had never 

 seen a mirror before, would naturally refer the sensation of which he 

 was the subject, to the influence of an object actually existing behind 

 the mirror : but when a very little knowledge was obtained, such 

 reference would no longer be made. And here let me remark 

 that it is not true that, in matters of vision, we ever labour under 

 illusions which refuse to be dispelled. When a child or savage sees 

 an object reflected from a mirror, and concludes that the remote 

 stimulus of vision is behind the mirror, two things must be distin- 

 guished : first, the image formed, in other words, the subjective 

 phenomenon produced, or the consciousness realized ; and secondly, 

 the inference drawn, viz : that an object exists behind the mirror. — 

 As far as the former of these is concerned, there is no illusion. The 

 image is apprehended as it really is. To deny this, would be to say 

 that a perception might be what it is not. In the latter point — the 

 inference drawn — there is certainly room for mistake or illusion ; but 

 the erroneous inferences of an uninstructed observer are capable of 

 being corrected. 



I shall only add, that, should the views advanced in this paper 

 prove to be well founded, they must materially affect the conclusions 

 at which we arrive on some questions which have recently excited 

 considerable discussion. I refer to the principles involved in the 

 construction of the Stereoscope; to our (so-called) perception of 

 relief; to the curious changes which often seem to take place in the 

 solids represented by plane outline figures ; and to other matters of 

 like description, into the particular discussion of which it would be 

 beyond our present purpose to enter. 



