THE DIOPTRIC SYSTEM. 



1031 



through the nodal point and the centre of rotation of the eyeball. If 

 the different refractive surfaces were exactly centred, as we shall see is 

 not the case, it would pass through the summits of the three chief curved 

 surfaces. The line of vision of is the line passing from the fovea 

 centralis through the nodal point of the reduced eye, and continued 

 to the point of fixation external to the eyeball. The line of fixation 

 om is the line passing from the external fixation point to the centre 

 of rotation of the eyeball. A fourth line, called by Helmholtz x the 

 Visirlinic, passes from the point of fixation to the centre of the 

 pupil ; owing to the frequent eccentricity of the pupil, this may differ 

 considerably from the lines of vision and fixation. 



The cornea is ellipsoidal, 2 and the major axis of the corneal ellipse, 

 el, may not correspond to the summit of the cornea. The angle 

 between the line of vision and the major axis of the corneal ellipse is 

 known as the angle a (oxe in Fig. 376) ; the angle between the optic 

 axis and the line of fixation, as the angle y. 



The deviation of the line of vision from the optical axis, and the imper- 

 fect centring of the eye, are shown by the following experiment of 

 Helmholtz. 3 In Fig. 377, ab is a graduated scale with apertures at a and 

 b ; the observer at a sees the three 

 images of a light at b, reflected from 

 cornea and lens of an eye at d, If 

 the line of vision corresponds with 

 the optical axis, the observer should 

 see the anterior lens image midway 

 between the others, when the observed 

 eye is directed to the centre of the 

 scale, c. It is found that the point of 

 fixation has to be shifted to one side, 

 i.e. the nasal side of cd, and slightly 

 above the plane abd. If the eye were 

 properly centred, and the observer and 

 light at a and b exchanged places, the 

 anterior lens image should still be mid- 

 way between the others. In none of 

 the eyes examined by Helmholtz was 

 this the case, and this observation 

 has been confirmed by Tscherning. 4 



As shown by the experiment described, the optic axis cuts the 

 posterior pole of the eyeball on the inner side of the yellow spot and 

 slightly above it. The angles a and y may be determined with the 

 ophthalmometer. A light to serve as object for the reflected image is 

 placed over the instrument, which is furnished with a graduated arc 

 and movable fixation point. The position of the eyeball is found 

 which gives an image exactly in the centre of the cornea, i.e. the cornea 



1 "Handbuch d. physiol. Optik," 2te Aufl., Hamburg u. Leipzig, 1896, S. 115. 



2 The generally accepted view, that the cornea is ellipsoidal, has been denied by Sulzer 

 (Arch, d'opht., Paris, 1891, tome xi. p. 419) and others. The form of the surface has been 

 determined by calculation from observations of the curvature at a certain number of points. 

 From examination of a large number of points of the corneal surface with the ophthalmo- 

 meter of Javal and Schibtz, Sulzer came to the conclusion that the central part is approxim- 

 ately spherical ; that at a certain distance from the centre the cornea becomes flatter, the 

 nasal side more than the temporal, and the upper more than the lower half. 



3 "Handbuch d. physiol. Optik," 2te Aufl., S. 108. 



4 Ztschr. f. Psychol, u. Physiol, d. Sinmsonj., Hamburg u. Leipzig, 1892, Bd. iii. S. 475. 



