374 VISION. LECT. XX. 



At the point F draw a perpendicular, cc', at the axis, to 

 the rays B/and x'f produced. At the pointy, also draw a 

 perpendicular c d at the axis, to the extreme reflected rays 

 AF and A'F. 



If, now, we consider a luminous ray of light traversing 

 the pupil at any point M, situated beyond the planes AOA', 

 BOB', this ray will no longer meet the axis of the eye, but 

 will be refracted so as to rest at the same time upon the 

 line cfc' and upon the line CFC/. Hence, it follows that: 



The luminous fasciculus which falls on the surface of the 

 cornea parallel to the axis will be so refracted, that in the 

 whole extent of the focal interval, jfr, it will form a very 

 narrow and very concentrated fasciculus, surrounding the 

 axis on all sides, and terminating very near it by a twisted 

 surface (surface gauche.) 



It is within the focal interval, between the points y and 

 F, at the point R, for instance, that the retina is placed. 

 Hence the refracted fasciculus depicts, on the surface of the 

 retina, a very narrow elliptic surface around the axis, and 

 on which all the rays which have traversed the opening of 

 the pupil meet. 



It follows, then, that a luminous point, placed before the 

 eye, does not meet upon the retina at a single point, but 

 upon a very small surface, proceeding from the meeting of 

 the retina and of the fasciculus concentrated about the axis 

 in the focal interval /F. 



Let us suppose, now, that the exterior point recedes from 

 or approaches the eye, the entire focal interval ft will at the 

 same time change its place, so that the retina which at first 

 was at R will be at R", or at R', being always contained be- 

 tween the points f and F. Hence, it follows, that this re- 

 tina will be always met by the concentrated fasciculus 

 around the axis in the focal interval, and that the surface 

 of intersection of this fasciculus, and of the nervous mem- 



