62 AN ESSAY ON 



of which it is situated, is prevented from oc- 

 curring, by a tendency to a change of its visible 

 direction the contrary way, produced by the 

 muscular actions which regulate the mutual 

 distance of the pupils. To know how this 

 happens, suppose the two optic axes to be pa- 

 rallel to each other, and perpendicular to the 

 visual base ; and let a physical line be placed 

 in either of them, so as entirely to coincide 

 with it. This line will, therefore, not only be 

 in reality perpendicular to the visual base, but 

 will, in the present state of things, likewise 

 appear so. Incline afterward both the axes 

 equally to the left side, and it is manifest that 

 the line coinciding, say, with the right axis, 

 must appear equally inclined. Let now the 

 right axis be kept fixed, and the left be carried 

 back again, and its motion continued, until it 

 be as much inclined toward the right side, as 

 itself was just before, and as the right axis is 

 still to the left side ; the consequence will be, 

 that the line in the right axis must again be 

 seen perpendicular to the visual base; for such 

 is the present position of the common axis. 

 Here then we have had two opposite causes of 

 change of apparent direction acting in succes- 

 sion. The muscular actions, producing the joint 

 motions of the eyes, first bent the visible posi- 

 tion of a line, in the right optic axis, from a 



