42 AN ESSAY ON 



of both eyes, to the left of the right axis, and to 

 the right of the left. Again, it has been shown 

 by the second proposition, that objects, placed 

 in the common axis, appear to each eye to be 

 situated in the axis of the other ; and the ter- 

 minations of both optic axes, at the visual base, 

 are distant half this base, from the similar ter- 

 mination of the common axis, the left being to 

 its right, and the right to its left. 



Let it now be supposed that two objects, one 

 placed in the axis of either eye, the right for 

 instance, and the other in the common axis, be 

 viewed at the same lime by that eye, it is evi- 

 dent that the visible directions of both will be 

 equally removed to the left, from their real posi- 

 tions. But such an alteration of visible direc- 

 tion, from real position, cannot be imagined to 

 happen, with respect to objects placed in the 

 optic and common axes, unless a similar effect 

 be, at the same time, produced upon such as are 

 situated any where between those lines, or in 

 their vicinity. Facts confirm this : If a line, for 

 example, be drawn through the intersection of 

 the optic axes to a point in the visual base, 

 exactly in the middle between the terminations 

 there of the right and common axes, its ap- 

 parent situation, to the right eye, will be found 

 to have the same relation to the apparent situa- 

 tions of lines placed in the right and common 



