DEFECTS IN THE OPTICAL APPARATUS 



68 5 



separated by a vertical line on the refractive surface, are focused at/ x , while 

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

 are 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, /. If the retina of the eye be 

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



FIG. 463. Astigmatic Charts. 



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/! 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 focused, 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 of rays or halos in the 

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

 dition 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/!, 

 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 

 further illustrated in figure 465 which represents the type of image formed 

 at the position/! shown in figure 464. 



Myopia. This is that refractive condition of the eye in which parallel 

 rays are brought to focus in front of the retina, 4, figure 466. It is due 



