360 SI R DAVID BREWSTER ON THE LAW OF VISIBLE POSITION 



eyes,* we shall find it difficult to believe that two similar unequal figures can 

 coalesce ; or that " similar pictures, falling upon corresponding points of the two 

 retinae, may appear double, and in different places." 



5. On the Cause of the Perception of Objects in Relief by the Coalescence of Dissimilar 



Pictures, 



Mr WHEATSTONE concludes his interesting paper with an inquiry into the 

 cause " mhy two dissimilar pictures, projected on the two retinae, give rise to the 

 perception of an object in relief." " I will not attempt," he adds, " at present, to 

 give the complete solution of this question, which is far from being so easy as at 

 a first glance it may appear to be, and is, indeed, one of great complexity. I 

 shall, in this place, merely consider the most obvious explanations which might 

 be offered, and shew their insufficiency to explain the whole of the phenomena." 



Mr WHEATSTONE then proceeds to describe the process of vision in the same 

 manner as we have done in \ 3 ; but impressed with the conviction that his previous 

 results are correct, he adds, " All this is in some degree true ; but were it entirely 

 so, no appearance of relief should present itself when the eyes remain intently 

 fixed on one point of a binocular image in the stereoscope." He then gives the 

 following experiment as decisive on the subject : " Draw two lines, about two 

 inches long, and inclined towards each other as in fig. 7, on a sheet of paper ; and 

 having caused them to coincide by converging the optic axes to a point nearer 

 than the paper, look intently on the upper end of the resultant line, without al- 

 lowing the eyes to wander from it for a moment. The entire line mitt appear 

 single, and in its proper relief," &c. After making this experiment with the greatest 

 care, we admit that it may appear single, without being single. To us it does 

 not appear single, but exactly the same as a line having the same length and the 

 same position appears in ordinary vision. Now, though this latter line appears 

 single to most eyes, yet it is certain that every point of it is double and indistinct, 

 excepting the point on Avhich the attention is fixed, and to which the optic axes con- 

 verge. The vision of objects in relief from the union of dissimilar pictures, is per- 

 formed by the very same process as the vision of real objects in relief by the or- 

 dinary agency of our two eyes ; and in establishing this principle, the true cause 

 of the phenomenon discovered by Mr WHEATSTONE will be readily obtained. 



Mr WHEATSTONE considers it as experimentally established, " that the most 

 vivid belief of the solidity of an object of three dimensions arises from two diffe- 

 rent perspective projections of it being simultaneously presented to the mind ;" 

 and that " the simultaneous vision of two dissimilar pictures suggests the relief of 



* See Letters on Natural Magic. Lett. III., p. 54. 



