i o 3 2 VISION. 



is doubled by altering the plates till the reflected images of the flame 

 fall on opposite edges of the cornea, as shown in Fig. 374. The position 

 of the fixation point on the graduated arc will give the angle 7, and the 

 angle may be calculated, since in Fig. 378, vc, the distance from the 

 centre of rotation to the graduated arc, is known, and also kc, the distance 

 from the nodal point to the centre of rotation. The method is not 

 strictly accurate, for the centre of the cornea does not, as already men- 

 o tioned, necessarily correspond with 



the corneal axis, and modifications 

 of the method have been devised 

 to overcome this difficulty. 1 



According to Donders, 2 in em- 

 metropic eyes the angle a is about 

 5, so that, when the lines of vision 

 are parallel, the optic axes would 

 diverge 10. The deviation was 

 found to be greater in hyperme- 

 tropic, and less in myopic eyes, in 

 which the deviation may be nil, and 



the optic axes may even converge. If the optic axis and the axis of the 

 corneal ellipse coincide, the angles a and 7 vary together, and the difference 

 between them diminishes, the further the point of fixation is from the eye. 



In addition to the three reflected images used in the ophthalmometric 

 methods, a fourth may be observed reflected from the posterior surface of the 

 cornea. It may be seen by placing the flame well on one side, and magnifying 

 the corneal image with a lens. 



An entoptic image may also be seen, owing to the fact that a certain amount 

 of the light reflected from the posterior surface of the lens is reflected again 

 from the posterior surface of the cornea. The light from the anterior surface 

 of the lens comes to a focus near the posterior surface of the lens, 3 and, owing 

 to its distance from the retina, is not observed entoptically. The light from 

 the posterior surface of the lens, when again reflected, comes to a focus near the 

 retina, and gives rise to an entoptic image. When a flame is held on one side 

 of the line of vision in a dark room, a faint inverted image may be seen on the 

 opposite side of the line of vision, which moves in the opposite direction to 

 that of the flame. This image may in some cases be the basis of the occur- 

 rence of monocular diplopia. 



Myopia and hypermetropia. Theoretically, myopia might be due 

 to the curvature of any of the refractive surfaces being greater than 

 normal, to increase of the refractive index of one of the media, or to 

 elongation of the eyeball. Similarly, hypermetropia might be due to 

 diminished curvature, diminished refractive index, or to shortening of 

 the eyeball. In nearly all cases, change in the shape of the eyeball is 

 believed to be the most important factor, although altered curvature 

 may account for many slight examples of these defects. It has been 

 suggested that cases of transitory myopia in certain inflammatory con- 

 ditions of the eyeball may be due to increased refractive index of the 

 aqueous. The refraction may differ in the two eyes, and this condition 

 is known as anisometropia. 



Astigmatism. This term is employed for two kinds of defect, 



1 Arch.f. OpMh., 1865, Bd. xi. Abth. 2, S. 257. 



2 "Anomalies of Accommodation and Refraction," 1864, p. 183. 



3 Ztschr.f. PsychoL u. Physiol. d. Sinnesorg., Hamburg u. Leipzig, 1892, Bd. iii. S. 429. 



