VISION 



1021 



the phakoscope (Practical Exercises), the corneal image is unchanged 

 in size, brightness, and position. The middle image diminishes 

 in size, comes forward, and moves nearer to the corneal image. 

 This shows that the curvature of the anterior surface of the lens 

 has been increased that is to say, its radius of curvature diminished 

 for the size of the image of an object reflected from a convex 

 mirror varies directly as the radius of curvature. A slight change 

 takes place in the image from the posterior surface of the lens, 

 indicating a small increase of its curvature too. By means of a 

 method founded on the observation of the changes in these images, 

 and a special instrument called an ophthalmometer which allows 

 of their measurement, Helmholtz 

 has calculated that during maxi- 

 mum accommodation, the radius 

 of curvature of the anterior surface 

 of the lens is only 6 mm., as com- 

 pared with 10 mm. when the eye 

 is directed to a distant object and 

 there is no accommodation. When 

 the lens has been removed for 

 cataract, fairly distinct vision may 

 still be obtained by compensating 

 for its loss by convex spectacles 

 of suitable refractive power (10 

 diopters* for distant vision, and 

 15 diopters for the distance at 

 which a book is usually held), but 

 no power of accommodation re- 

 mains. The person does indeed 

 contract the pupil in regarding a 

 near object, just as happens in the 

 intact eye; the most divergent 

 rays are thus cut off and the image 

 made somewhat sharper, and there 

 may appear to be some faculty of 

 accommodation left. But the loss 

 of the whole iris by operation does 

 not affect accommodation in the least; the iris, therefore, takes 

 no part in it. That no change in the antero-posterior diameter 

 of the eyeball, caused by its deformation by the contraction of the 

 ^extrinsic muscles, can have any share in accommodation, as has 



* A diopter (i D.) is the unit of refractive power generally adopted in 

 measuring the strength of lenses, and corresponds to a lens of i metre focal 

 length. A lens of 2 diopters (2 D.) has a focal length of metre, a lens of 

 4 diopters (4 D.) a focal length of J metre, and so on. The diverging power 

 of concave lenses is similarly expressed in diopters with the negative sign 

 prefixed. Thus, a concave lens of i metre focal length has a strength of i D.. 

 and will just neutralize a convex lens of i D. 



Fig. 421. Purkinje - Sanson Images. 

 A, in the absence of accommoda- 

 tion; B, during accommodation for 

 a near object. The upper pair of 

 circles enclose the images as seen 

 when the light falls on the eye. 

 through a double slit on a pair of 

 prisms; the lower pair show the 

 images seen when the slit is single 

 and triangular in shape. 



