IN SINGLE AND BINOCULAR VISION. 359 



with both eyes open ; and hence it is that, in very oblique vision, one of the eyes 

 resigns its office, and leaves the other to view the object distinctly and singly.* 



But, in point of fact, AB, fig. 7, does not coalesce with CD. If the eye strives 

 to see distinctly any object at the point S', then AB coalesces with m n. If the eye 

 looks fixedly at C when A is united with C, AS' will unite with CS' and S'B with 

 S'n ; and if the eye is fixed intently on D when B and D are united, S'B will coa- 

 lesce with S'D and AS with m S'. In these two last cases, the coalescence arises 

 from the same cause as the coalescence of dissimilar forms in Mr WHEATSTONE'S 

 fundamental experiment, as I shall now shew. 



2. If we join C m, D n, fig. 7 (as is done in fig. 8), we may regard AB and 

 C m S' n D as dissimilar images of a solid, consisting of two triangles C m S', D n S', 

 united at their apex. In this case, AS, fig. 8, will coincide with OS' and S'B with 

 Sw. If the two dissimilar images are, as in figs. 9, 10, 11, and 12, AB will not 

 appear to coalesce with CD. In fig. 13, the coalescence is not complete ; but it 

 becomes so by removing the portion a b of the line AB the part A a coalescing 

 with C, and b B with D. In fig. 14, the line AB will not coalesce with CD ; but 

 each separate portion of AB will, when the other two portions are concealed or 

 removed, coalesce with the corresponding portion of CD. 



The ocular equivocation, as it may be called, which is produced by the capri- 

 cious disappearance and reappearance of images formed on nearly corresponding 

 parts of the retina of each eye, is placed beyond a doubt by Mr WIIEATSTONE'S 

 own experiments. f Having inscribed the letters A, S, fig. 15, in two equal cir- 

 cles, he unites the circles, and finds, that, while 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 re- 

 placed by the entire letter." I have long agot described an affection of the re- 

 tina, 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 con- 

 templation of the red one, the green pattern will sometimes vanish entirely, leav- 

 ing 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 disappearance of objects seen out of the axis of vision by one or by both 



* The fact of objects seen obliquely not being double, is ascribed by Mr WHEATSTONE to the coa- 

 lescence of the images of different magnitudes given by each eye. 

 t Phil. Trans. 1838, p.386, 14. 

 } Letters on Natural Magic, p. 54. 



