ASTIGMATISM. 93 



curvature of the cornea of about -J- of an inch, and have a 

 single optical centre situated of an inch back of the cornea, 

 the " principal point " being in the cornea at the axis of vision. 

 The posterior focal distance, that is, the focus at the bottom 

 of the eye of rays that are parallel in the air, is about of an 

 inch. The anterior focal distance, that is, of rays parallel in 

 the vitreous humor, is about f of an inch. The measure- 

 ments in this simple eye can be easily remembered and used 

 in calculations. 



Astigmatism. 



We have already alluded to an important peculiarity in 

 the optical apparatus ; viz., that the visual line does not coin- 

 cide exactly with the axis of the eye. 1 There is still another 

 normal deviation from mathematical exactness in the refrac- 

 tion of rays by the cornea and the crystalline lens, which is of 

 considerable importance. If we place before the eyes two 

 threads crossing each other at right angles in the same plane, 

 one of these threads being vertical, and the other, horizontal, 

 when the optical apparatus is adjusted so that one line is seen 

 with perfect distinctness, the other is not well defined. In 

 other words, when we accommodate for the vertical thread, 

 the horizontal is indistinct, and vice versa. If the horizontal 

 line be seen distinctly, in order to see the vertical, without 

 modifying the accommodation, it must be removed to a 

 greater distance. This depends chiefly upon a difference in 

 the vertical and the horizontal curvatures of the cornea, so 

 that the horizontal meridian has a focus slightly different 

 from the focus of the vertical meridian. A condition oppo- 

 site to that observed in the cornea usually exists in the crys- 

 talline lens ; that is, the difference which exists between the 

 curvatures of the lens in the vertical and the horizontal me- 

 ridians is such that the deepest curvature in the lens is situated 

 in the meridian of the shallowest curvature of the cornea. 

 In this way, in normal eyes, the aberration of the lens has a 



1 See page 72. 



