102 J. LeConte — Phenomena of Binocular Vision. 



viders so that the points are exactly separated by that distance 

 and then holding it level touch the two retinae any where, the 

 points touched will be corresponding points. This distance is 

 called the interocular base. It varies in different eyes; in my 

 own it is exactly 62 mm . It is probable that the two retinae thus 

 correspond rod for rod and cone for cone. The central spots are 

 par excellence corresponding points. 



Now the law asserts that impressions on corresponding points 

 are referred outward to the same point in space and therefore 

 seen single, while impressions on non-corresponding points are 

 referred outward to different points in space and therefore seen 

 double. Furthermore — and this is the important point in the 

 subsequent explanation — if the non-corresponding points im- 

 pressed be farther apart than the distance between correspond- 

 ing points or than the interocular base, then the double images 

 are heteronymous, i. e. the left image belongs to the right eye 

 and the right image to the left eye ; but if the impressed points 

 are nearer together than corresponding points, then the double 

 images are homonymous, i. e. are on the same side as the eye 

 to which they respectively belong. Now, in the former case, 

 i. e. when the impressed points are too far apart, the impress- 

 ing object is nearer than the point of sight; while in the latter 

 case, i. e. when the impressed points are too near together, the 

 impressing object is further off than the point of sight. 



Application. — We are now prepared to explain the last exper- ' 

 iment. In fig. 2, PP is a section of a tessellated plane, and 

 a, 5, c, represents the position of the regular figures; the 

 optic centres or nodal points of the two eyes and EE portions 

 of the two retinas. The eyes are supposed to be fixed steadily 



on bb, which therefore impress corresponding points, viz.: the 

 central spots bb and consequently are seen combined as one at 

 the point of sight B. At the same time a a and cc also are 

 supposed to combine and by geometrical construction would be 



