28 A TYPICAL VERTEBRATE EYE: THE HUMAN 



spectacle lens. As we shall see later, all three of these conditions which 

 for the human eye are 'errors', are perfectly normal and desirable situ- 

 ations in the eyes of various vertebrates whose visual requirements differ 

 greatly from our own. 



Fig. 13 — Astigmatism. 



a, a square piece of normal cornea whose radius of curvature, r, is the same in all meridians, 

 images a point p as a point on the screen s. In any other position the screen would intercept 

 a blur-square. 



b, a piece of cornea whose radius of curvature in one direction, /, exceeds its radius of 

 curvature in another direction, r, is said to be astigmatic. It images a point p as a line 

 (horizontal in this instance) ih on a screen s placed in its first focal plane, and also as a 

 line at right angles to the first (the linear vertical image iv) on a screen /' placed in its 

 second focal plane. The most compact image of p is the 'figure of least confusion', flc, on the 

 screen /; but this image is a blur-square — the point p is nowhere imaged as a point, as in a. 



c, the same piece of astigmatic cornea as in b sharply images the horizontal limbs of a cross 

 on the screen s, places a blurred cross on the screen /, and sharply images the vertical limbs 

 of the cross on the screen s". The whole of the objea cannot be sharply imaged at any one 

 distance from the astigmatic refracting structure. 



