SINGLE VISION. 47 



of the optic axes and any given object, to the 

 visual base, which is to be produced, if neces- 

 sary ; and let it be called the line of the ob- 

 ject's real position. Take afterward, in the 

 visual base, or its production, two points, one 

 on each side of the line of real position, and 

 both distant from its termination there, half 

 the visual base. Lines drawn from these points, 

 through the point of intersection of the optic 

 axes, must consequently contain the two visible 

 positions of the object. But when this is 

 situated in the horopter, the line of real posi- 

 tion will coincide with the horopter, and will 

 not therefore reach the visual base, unless at 

 an infinite distance from the eyes. For which 

 reason, the two lines, containing the visible po- 

 sitions of the object, must fall upon the visual 

 base at a like distance, and must consequently 

 be regarded as coinciding with each other. 

 When the object is not in the horopter, the two 

 lines of visible direction will be found, by the 

 same means, not to coincide. 



That I might simplify a matter, which under 

 my management, must, I fear, still be of dif- 

 ficult apprehension, I have, in expressing the 

 law of vision, so frequently mentioned, pur- 

 posely confined it to objects situated in the 

 plane of the optic axes. But in persons who 

 do not squint, or whose eyes are not distorted 



