670 SIR DAVID BREWSTER ON THE KNOWLEDGE OF 



than the interval of your eyes, with your arm extended, hold the head or joint 

 in the ball of your hand with the points outwards, and equidistant from your 

 eyes, and somewhat higher than the joint. Then, fixing your eyes upon any remote 

 object lying in the plane that hisects the interval of the points, you will first per- 

 ceive two pair of compasses (each by being doubled with their inner legs crossing 

 each other, not unlike the old shape of the letter W.) But by compressing the 

 legs with your hand, the two inner points will come nearer to each other ; and 

 when they unite (having stopt the compression), the two inner legs will also 

 entirely coincide and bisect the angle under the outward ones, and will appear 

 more vivid, thicker and larger, than they do, so as to reach from your hand to the 

 remotest object in view even in the horizon itself, if the points be exactly co- 

 incident."* Owing to his imperfect apprehension of the nature of this pheno- 

 menon, Dr SMITH has omitted to notice that the united legs of the compasses lie 

 below the plane of A B C, and that they never can extend farther than the bin- 

 ocular centre at which their points A and B are united. 



There is another variation of these experiments which possesses some inte- 

 rest, in consequence of its extreme case having been made the basis of a new 

 theory of visible direction by the late Dr WELLS.| Let us suppose the eyes of the 

 observer to advance from E to N, and to descend along the opposite quadrant 

 on the left hand of N G, but not drawn in fig. 3 (plate 17), then the united image 

 of A C, B C, will gradually descend towards C G, and become larger and larger. 

 When the eyes are a very little above the plane of A B C, and so far to the 

 left hand of A B, that C A points nearly to the left eye, and C B to the right eye, 

 then we have the circumstances under which Dr WELLS made the following ex- 

 periment : " If we 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 two edges ivill 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 

 ABC, 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 



* SMITH'S Optics, vol. ii. p. 388, 977. t Essay on Single Vision, &c., p. 4i. 



