32 AN ESSAY ON 



of the visual base, through the point of inter- 

 section of the optic axes, or parallel to them, if 

 they be parallel to each other, I name it the 

 common axis*. This term, I believe, was in- 

 vented by Alhazen ; but with him it signified a 

 line drawn from the centre of the junction of 

 the optic nerves, through the middle of the in- 

 terval between the centres of the retinas. Such 

 a line was consequently immoveable. As the 

 term, however, is not in modern use, no mis- 

 take can arise from confounding the two mean- 

 ings, and the reason will soon be seen, why I 

 employ it in the sense I have mentioned. Those 

 who are acquainted with the writings of the 

 older opticians, will perceive, that I give it 

 nearly the same signification as they did to their 

 common radius. 



* It may be said, perhaps, that as I do not define the 

 points of the corneas, upon which the optic axes fall, I can- 

 not, with propriety, desire the line which connects them to be 

 divided. To this I answer, that it is not necessary for the 

 purpose I have mentioned, that they should be defined ; if it 

 be granted to me, and I think it cannot be refused, that upon 

 whatever point of the right cornea the right axis falls, the 

 left axis will fall upon a similarly situated point of the left 

 cornea ; that is, if this point of the right cornea be at any 

 given distance from its middle, and upon the inside of it, the 

 corresponding point of the left cornea will be at the same 

 distance from the middle of this, and also upon its inside. 

 Whatever extent, therefore, the line connecting these places 

 of the corneas may have, its middle point will be the same. 



