SINGLE VISION. 31 



"these objections ; and such a theory, I hope, I 

 shall bring forward in the following proposi- 

 tions, after mentioning the meanings which I 

 &ffix to several terms I shall frequently employ. 



EXPLANATION OF TERMS. 



I. When a small object is so placed with re- 

 spect to either eye, as to be seen more distinctly 

 than in any other situation, I say it is then in 

 the optic axis, or the axis of that eye ; and if 

 another small body be interposed between the 

 former and the eye, so as to conceal it, and if 

 a line joining the two be produced till it falls 

 upon the cornea, I call this line the optic axis> 

 or the axis of the eye ; leaving for future de- 

 termination the precise point of the cornea it 

 falls upon, or what part of the retina receives 

 the picture of an object which is placed in it. 



II. When the two optic axes are directed to 

 a small object not very distant, they may be 

 conceived to form two sides of a triangle, the 

 base of which is the interval between the points 

 of the corneas, where the axes enter the eyes ; 

 but if the object be very distant, then they may 

 be supposed to be two sides of a parallelogram, 

 whose base is the same interval. To avoid cir- 

 cumlocution, I shall call this interval the visual 

 base. 



III. If there be drawn a line from the middle 



