650 



THE SENSES 



courses through eyes which have this defect, but an examination of figure 

 464 will show their refraction. In this figure four rays coming from the 

 point L in the arrows are represented as striking on the refractive surface 

 of the eye atA,B, C, D, and being converged toward a focus. The rays A, C, 

 separated by vertical line on the refractive surface, are focussed at f lt while the 

 lines A, B, separated by the horizontal distance on the refractive surface, are 



ABC 

 FIG. 463. Astigmatic Charts. 



brought to a focus at / 2 . The point L, therefore, has two apparent focal points, 

 one point composed of the rays that strike in a horizontal plane, / 2 , the other 

 of rays that strike in a vertical plane, / r If the retina of the eye be placed 

 at /! it will see an image of a point with indistinct horizontal rays. If placed 

 at the position / 2 it will see a luminous point with indistinct rays in the 

 vertical plane* If the series of points in the arrow MN be considered, 

 it is evident that at the position f l the rays which fall in the vertical plane 

 will form distinct foci, while those that fall in the horizontal plane will form 

 overlapping diffuse images in that plane. Since they are overlapping, they 

 will not appear separate except at the ends of the image of the arrow, and the 

 arrow will therefore be seen distinctly. If the position / 2 is considered 

 where the rays of the horizontal plane are focussed, then it is evident that 



FIG. 464. The Unequal Refraction of Rays in an Astigmatic Eye. (John Green.) 



the points in the arrow MN will present a series cf rays or halos in the 

 vertical plane, thus rendering its outline very dim or indistinct. The condi- 

 tion with the arrow OP is exactly the reverse. Hence, in the astigmatic 

 eye the images of the horizontal arrow MN will be distinct at the focus } ly 

 while the image of the vertical arrow OP will be distinct in the focus / 2 , and 

 the eye cannot see the two lines distinctly at the same time. This condition is 



