440 Sir David Brewster on the Law of Visible Position 



union takes place; and the eye must recover its ordinary 

 functions as instantaneously, for the moment we intercept one 

 of the images the other resumes its proper size. 



In order to understand what the nature of this supposed 

 change actually is, let MN, M'N' (fig. 3, 4) be the two eyes, 



Fig. 3. Fig. 4. 



A B the larger image, and a b the smaller one ; then if C be 

 the centre of curvature of the retina, the points A, B will be 

 seen in the directions An, Bm intersecting at C, and the 

 points a, b in the directions as, br intersecting at C But 

 when these separate images coalesce, in consequence of A B 

 becoming less and b c greater, the points A, B, a, b must be 

 seen in the directions A n, B o, a v, b t, intersecting at new 

 centres of visible direction c, d, the one further from, and the 

 other nearer to, the retina. If we now shift the larger picture 

 to the right eye, and the smaller to the left eye, the function 

 of the retina will be again changed : the left eye M N will 

 have its lines of visible direction as in fig. 4, and the right eye 

 as in fig. 3. Such an oscillation of the binocular centre of 

 visible direction on each side of the monocular centre, produced 

 solely during the attempt to unite unequal images, would in- 

 dicate a function of the retina so extraordinary, that the most 

 incontrovertible experiments, and the universal experience of 

 accurate observers, could alone give it credibility. 



There is no doubt that the two unequal images appear to 

 coalesce; but if we make the outlines of the squares and 

 circles luminous, by pricking small holes in their outlines, and 

 exposing them to very strong light, we shall find it impossible 

 to produce a coincidence. The best way to make this expe- 

 riment is to take two lines, A B, a b, fig. 5, of unequal lengths, 

 and with a large pin to perforate the lines at A, B, a, b, so that 

 when we attempt to unite them, as at fig. 6, we shall see with 

 perfect distinctness their four luminous extremities. When 



