314 Sir D. Brewster on the Knoidedge of 



hold two thin rules in such a manner that their sharp edges 

 (A C, B C in fig. 3) shall be in the optic axes, one in each, 

 or rather a little below them, the txw edges roill be seen united 

 in the common axis (G C in fig. 3); and this apparent edge 

 will seem of the same length with that of either of the real 

 edges, when seen alone by the eye in the axis of which it is 

 placed." This experiment, it will be seen, is the same with 

 that of Dr. Smith, with this difference only, that the points of 

 the compasses are directed towards the eyes. Like Dr. Smith, 

 he has omitted to notice that the united image rises above 

 G H, and he commits the opposite error of Dr. Smith, in 

 making the length of the united image too short. 



If in this form of the experiment we fix the binocular centre 

 beyond C, then the united images of A C, B C descend below 

 G C, and vary in their length and in their inclination to G C, 

 according to the height of the eye above the plane of A B C, 

 and its distance from A B. 



It is a remarkable circumstance, that no examples have 

 been recorded of false estimates of the distance of near objects, 

 in consequence of the accidental binocular union of similar 

 images. This has, no doubt, arisen from the rare occurrence 

 of these circumstances or conditions, under which alone such 

 illusions can be produced. In a room where the paper-hang- 

 ings have a small pattern, or similar figures recurring at the 

 distance of 1, 1|, or 2 inches, a short-sighted person might 

 very readily turn his eyes on the wall, when their axes con- 

 verged to some point between him and the wall, which would 

 unite one pair of the similar images ; and in this case he would 

 see the wall nearer him than the real wall, and moving with 

 the motion of his head like something aerial. In like manner, 

 a long-sighted person, with his optical axes converged to a 

 point ijeyond the wall, might see an image of the wall more 

 distant, and of an aerial character : — or a person who has 

 taken too much wine, which often fixes the optical axes in 

 opposition to the will, might, according to the nature of his 

 sight, witness either of the illusions above-mentioned. 



In the preceding observations, we have confined ourselves 

 to the binocular union of figures upon an opake ground. This 

 limitation almost necessarily precluded us from observing the 

 results when the binocular centre is beyond the pinne where 

 these figures are situated, because it is not easy to adjust the 

 eyes to a distant object, unless we look through the surfaces 

 containing the figures. Now this is by far the most interest- 

 m<r form of the experiment, and it has the advantage of putting 

 scarcely any strain upon the eyes, not only because the bino- 

 cular centre is more distant, but because we cannot, in this 



