442 Sir David Brewster on the Law of Visible Position 



uniting with the similarly strong line C D ; but if m n is a 

 strong line and C D a weak one (fig. 1 1), or an interrupted one, 

 AB will unite with mn, and not even apparently with CD. 

 In like manner, if A B be a weak line, it will unite with the 

 weak line mn rather than with C D. (See fig. 12.) 



Now, the apparent coalescence of similar lines arises from 

 the fact, that when corresponding, or nearly corresponding, 

 parts of the retinae are impressed with similar images, one of 

 the two more readily vanishes, independent of its liability to 

 vanish from its being out of the axis of vision. Whenever 

 two images interfere with one another so as to impede vision, 

 one of them disappears — or rather, is not taken cognisance of 

 by the eye. Hence it is that many sportsmen shoot 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, A B, fig. 7, does not coalesce with 

 C D. If the eye strives to see distinctly any object at the 

 point S', then A B coalesces with m n. If the eye looks fix- 

 edly at C when A is united with C, A S' will unite with C S' 

 and S' B with S' n ; and if the eye is fixed intently on D when 

 B and D are united, S' B will coalesce with S' D and A S with 

 m S'. In these last two cases, the coalescence arises from the 

 same cause as the coalescence of dissimilar forms in Mr. 

 Wheatstone's fundamental experiment, as I shall now show. 



2. If we join Cm, Dw, fig. 7 (as is done in fig. 8), we may 

 regard A B and CmS'»D as dissimilar images of a solid, 

 consisting of two triangles C m S', D n S', united at their apex. 

 In this case, A S, fig. 8, will coincide with C S' and S' B with 

 Sn. If the two dissimilar images are, as in figs. 9, 10, 11, 

 and 12, A B will not appear to coalesce with C D. In fig. 13 

 the coalescence is not complete ; but it becomes so by removing 

 the portion a b of the line A B, the part A a coalescing with 

 C, and b B with D. In fig. 14, the line A B will not coalesce 

 with C D : but each separate portion of A B will, when the 

 other two portions are concealed or removed, coalesce with 

 the corresponding portion of C D. 



The ocular equivocation, as it may be called, which is pro- 

 duced by the capricious disappearance and I'eappearance of 

 images formed on nearly corresponding parts of the retina of 

 each eye, is placed beyond a doubt by Mr. Wheatstone's own 

 experiments f. Having inscribed the letters A, S, fig. 15, in 

 two equal circles, he unites the circles, and finds, that, while 



* The fact of objects seen obliquely not being double, is ascribed by Mr. 

 Wheatstone to the coalescence of the images of different magnitudes given 

 by each eye. 



f Philosophical Transactions, 1838, p. 386, § 14. 



