to 3 4 VISION. 



is very slightly altered. This may be observed by means of the reflected 

 images seen with the phakoscope, and the amount of change may be 

 accurately measured with the ophthalmometer. The central point of 

 the anterior surface is found to move forwards, the amount of movement 

 averaging '5 mm. The radius of curvature of the anterior surface 

 diminishes from an average of 10 mm. to one of 6 mm., while in cases of 

 hypermetropia the increase of curvature may be even greater. Knapp l 

 found that in four individuals the amount of accommodation, calculated 

 from the change of curvature of the lens, agreed fairly closely with the 

 actually observed range of accommodation. 



According to Tscherning, 2 however, the increase of curvature does not 

 take place uniformly over the whole anterior surface, but is greater for 

 its central part. He came to this conclusion after repeating an 

 experiment of Thomas Young, in which he used a plano-convex lens 

 with squares marked out on the plane surface, — -the aberroscope. On 

 holding this 10 cm. from the eye, and looking at a light, most individuals 

 see the outer lines curved with the convexity inwards, while, on accom- 

 modating, the curved lines become straight or even curved outwards. 

 Tscherning regards this as showing that in its resting state the 

 refraction is greater at the periphery of the lens than at the centre, 

 while during accommodation the increase of refraction is relatively less 

 at the periphery, so that the whole lens may become equally refractive, 

 or the central part even more refractive than the periphery. With 

 more exact measurements, by means of his ophthalmophakometer, 

 Tscherning found that while the radius of curvature of the central 

 parts altered from 10 mm. to 5 or 6 mm. during accommodation, a 

 point on the periphery, 14 mm. from the centre, only showed a radius of 

 8 mm. during accommodation, while in the more peripheral parts there 

 was actual and not merely relative flattening. 3 Crzellitzer 4 has since 

 repeated and confirmed these observations. 



The curvature of the posterior surface is found to be very slightly 

 increased, the observations of numerous workers showing a change 

 from about 6 mm. to 5 mm. in radius. 



Most investigators have found no alteration in the position of the 

 lens, i.e. the position of the posterior surface. Eeich 5 and Tscherning 6 

 have, however, each described slight recession of the posterior surface, 

 about -1 mm., and Hensen and Voelckers 7 observed recession of the lens 

 in animals, shown by the movements of a style placed against its posterior 

 surface. _ Tscherning also described a slight movement downward of the 

 lens during accommodation. The question has been very thoroughly 

 investigated by Hess, 8 who finds that, when the eye is accommodated, 

 there is considerable mobility of the lens. In cases of absent iris (after 

 iridectomy) under the influence of eserin, Hess observed a marked 

 shaking of the lens with every movement of the head, the phenomenon 

 disappearing after instillation of atropin. The movement was most 

 marked with the head inclined backwards. For observations on the 

 normal eye, Hess took advantage of the fact that eserin acts on the 



1 Arch./. Ophth., 1860, Bd. vi. Abth. 2, S. 1. 



* Arch, de physiol. norm, etpath., Paris, 1894, tome vi. p. 40. 

 Ibid., 1895, tome vii. p. 158. 



* Arch./. Ophth., 1896, Bd. xlii. Abth. 4, S. 36. 



•' Ibid., 1874, Bd. xx. Abth. 1, S. 207. fi hoc. cit. 



1 Centralbl.f. d. mcd. JVisscnsch. , Berlin, 1866, S. 721. 

 Arch. f. Ophth., 1896, Bd. xlii. Abth. 1, S. 288. 



