Distance given by Binocular Vision. 313 



grow largei* and larger, and approximate in direction to C G 

 as the eyes descend from E'" to M. When the eyes are near 

 to M, and a little above the plane of A B C, the line, when 

 not carefully observed, will have the appearance of coinciding 

 with C G, but stretching a great way beyond G. This extreme 

 case represents the celebrated experiment with the compasses, 

 described by Dr. Smith, and referred to by Professor Wheat- 

 stone. He* took a pair of compasses, which may be repre- 

 sented by A C B, A B being their points, A C B C their legs, 

 and C their joint; and having placed his eyes about E above 

 their plane, he made the following experiment : — " Having 

 opened the points of a pair of compasses somewhat wider than 

 the interval of your eyes, with your arm extended, hold the 

 head or joint in the ball of your hand with the points out- 

 wards, and equidistant from your eyes, and somewhat higher 

 than the joint. Then, fixing yoia- eyes upon any remote object 

 lying in the plane that bisects the interval of the points, you 

 will first perceive 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 stopped 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 coincident*." Owing to his imperfect 

 apprehension of the nature of this phfenomenon, 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 

 further than the binocular centre at which their points A and 

 B are united. 



There is another variation of these experiments which 

 possesses some interest, in consequence of its extreme case having 

 been made the basis of a new theory of visible direction by 

 the late Dr. Wellsf. 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, 

 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 experiment : — " If we 



• Smitli's 0|)tics, vol. ii. p. 388, § 977- 

 f Essay on .Single Vision, &c., p. 44. 



