i°3° 



VISION. 



eye, which is rotated till the hair covers first one and then the other 

 border of the cornea. The angle through which the eyeball has moved, 

 namely, the angle yxy in Fig. 375, may be read off on the scale, and is about 

 56°. The diameter of the cornea yjf may be found with the ophthal- 

 mometer by doubling the cornea, by altering the plates till the doubled 

 images from the centre of the cornea correspond with the edges of the 



doubled cornea (as in 

 Fig. 374, with the dif- 

 ference that the circles 

 represent the doubled 

 cornea, and not the 

 pupil). The angle yxz 

 and the side yz being 

 known, xz, the distance 

 from the centre of 

 rotation to the basis 

 of the cornea, may be 

 found, and the height 

 of the cornea, i.e. 

 2 "6 mm., being added, 

 will give the distance 

 of the centre of rotation behind the summit of the cornea. By this 

 method Bonders 1 found the distance in emmetropic eyes to average 

 13*54 mm. In myopic eyes the centre was found to be 14*52 mm., and 

 in hypermetropic 13 "22 mm. behind the anterior corneal surface. 



According to J. J. Miiller 2 and Berlin, 3 the centre of rotation is not a fixed 

 point, and recedes from the cornea when the lines of fixation are directed up- 

 wards. The former determined the path of a definite point of the cornea 

 during movement of the eyeball, while the latter arranged a number of fine 

 objects, so that they appeared in line with one another in different positions 

 of the eyeball, the point of section of the different lines corresponding with 

 the centre of rotation. 



Fig. 375.— Donders. 



Optical irregularities.— Before 



considering 



some further points 



Fig. 376. — Lan.lolt. 



about the refractive system, certain definitions will be necessary. The 

 optical axis of the eyeball (aa in Fig. 376) is the line which passes 



1 ic 



Accommodation and Refraction of the Eye," 1864, p. 181. 

 | Arch./. Ophth., 1868, Bd. xiv. Abtli. 3, S. 183. 

 "Ibid., 1871, Bd. xvii. Abth. 2, S. 150. 



