MYOPIA 



737 



composed of the unfocused rays of all other meridians than the vertical which 

 are in focus. If placed at the position/ 2 it will receive a luminous point with 

 indistinct halos 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 ver- 

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

 will form overlapping diffuse images in that plane. Since they are over- 

 lapping, 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 



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



considered where the rays of the horizontal plane are focused, then it is 

 evident that 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 

 condition 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 f, shown in figure 464. 



FIG. 465. Diagram of Character of Retinal Images in Astigmatism. (John Green.) 



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

 rays are brought to focus in front of the retina when the eye is at 

 rest, 4, figure 466. It is due either to an abnormal elongation of the 

 eyeball antero-posteriorly or to an increase in the convexity of the re- 

 fracting surfaces, or to both of these conditions. Parallel rays from a 

 distant point are focused in front of the retina, and, crossing, form circles 



