444 Sir David Brewster on the Law of Visible Position 



the common border remains constant, "the letter within it 

 will change alternately " from A to S. At the instant of 

 change the letter " breaks into fragments ; while fragments of 

 the letter which is about to appear, mingle with them, and are 

 immediately replaced by the entire letter." I have long ago* 

 described an affection of the retina, of an analogous kind, 

 which illustrates the subject under consideration. " If we 

 look very steadily and continuously with both eyes at a double 

 pattern — such as one of those on a carpet — composed of two 

 single patterns of different colours, suppose red and green ; 

 and if we direct the mind particularly to the contemplation of 

 the red one, the green pattern will sometimes vanish entirely, 

 leaving the red one alone visible; and, by the same process, 

 the red one may be made to disappear." When we join to 

 these various facts the remarkable phenomena of the disap- 

 pearance of objects seen out of the axis of vision by one or 

 by both eyesf, we shall find it difficult to believe that two si- 

 milar unequal figures can coalesce ; or that " similar pictures, 

 falling upon corresponding points of the two retinas, may ap- 

 pear 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 "why 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 con- 

 sider the most obvious explanations which might be offered, 

 and show their insufficiency to explain the whole of the phae- 

 nomena." 



Mr. Wheatstone then proceeds to describe the process of 

 vision in the same manner as we have done in § 3 ; but im- 

 pressed with the conviction that his previous results are cor- 

 rect, he adds, " All this is in some degree true', but were it en- 

 tirely 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 con- 

 verging the optic axes to a point nearer than the paper, look 

 intently on the upper end of the resultant line without allow- 

 * Letters on Natural Magic, p. 54. f Ibid., Let. III., p. 54. 



