834 ACCOMMODATION OF THE EYE. 



of the lens is nearer to the cornea. Neither of the other pairs (a, and c,) change 

 either in size or in position. By means of the ophthalmometer, the diminution 

 of the radius of curvature of the anterior surface of the lens during accommoda- 

 tion for near vision can be determined. 



2. As a result of the increased curvature of the lens during accommodation 

 for near vision, the refractive conditions within the eye must be changed. Ac- 

 cording to v. Helmholtz the measurements for the resting eye, and for the eye 

 accommodated for near vision are as follows (the first number is for the resting, 

 the second for the accommodated eye): Radius of the cornea 8 mm., 8 mm. 

 Radius of the anterior surface of the lens 10 mm., 6 mm. Radius of the posterior 

 surface of the lens 6 mm., 5.5 mm. Distance of the vertex of the anterior sur- 

 face of the lens from the vertex of the anterior surface 01 the cornea 3.6 mm., 

 3.2 mm.; of the vertex of the posterior surface of the lens 7.2 mm., 7.2 mm.; 

 of the anterior focal point 12.91 mm., 11.24 mm.; of the first principal point 

 1.94 mm., 2.03 mm.; of the second principal point 2.35 mm., 2.49 mm.; of the 

 first nodal point 6.96 mm., 6.51 mm., of the posterior focal point 22.23 mm., 

 20.25 mm., behind the anterior corneal surface. 



3. If the resting eye is viewed from the side, the pupil appears as a narrow 

 black streak. This becomes broader as soon as the eye is accommodated for 

 near vision, as the entire pupil moves forward. 



4. If light is thrown obliquely into the anterior chamber, the focal line formed 

 by the concave surface of the cornea falls upon the iris. If the experiment is made 

 with an eye arranged for distant vision, so that the focal line lies near the pupillary 

 margin of the iris, the line will recede immediately toward the scleral margin 



of the iris, as soon as the eye is accom- 

 modated for near vision, because the 

 iris is placed more obliquely, as its 

 pupillary margin moves forward. 



5. In accommodation for near vision 

 the pupil contracts; in distant vision 

 it dilates. The contraction takes place, 

 however, somewhat later than the 

 accommodation. This phenomenon may 

 be interpreted as an associated move- 

 ment, as both the ciliary muscle and the 

 FIG. 2 8 3 .-The Images of Purkinje-Sanson: a b c, sphincter of the pupil are innervated by 



in the eye at rest; a\ hi c\, in the eye accommo- the Oculomotor nerve. Examination of 



dated for near vision. Pi g 2gl will ghow that the sphincter 



may assist the muscle of accommodation 

 directly; for if the inner margin of the 



ins moves inward (toward r), this movement will be transmitted to the ciliary 

 margin of the choroid, which must likewise follow inward to some extent. The 

 movement of the choroid is, it is true, effected principally by the tensor of the 

 choroid. Accommodation is, however, still possible even when the iris is absent 

 or when its fibers are split. 



6. On rotation of the eyeball inward, the eye is accommodated involuntarily 

 for near vision. As both eyes rotate inward when the optic axes are directed 

 toward near objects, it is evident that the eye must be involuntarily accommo- 

 dated at the same time for near vision. 



7. The accommodation from a near to a far object (simple relaxation of the 

 tensor of the choroid) takes place more quickly than the reverse movement, 

 trom far to near. The process of accommodation requires a longer time the 

 nearer the object is brought to the eye. The time required for the image formed 

 oy the anterior surface of the lens to complete its change of position is less than 

 that required for subjective accommodation. 



8. When the eye is accommodated for a certain distance, it obtains a sharp 

 image not only of one point, but of a whole series of points behind one another. 

 I he line in which these points are situated is called the line of accommodation. 

 1 he farther away the point is for which the eye is accommodated the longer this 

 Jine becomes; beyond a distance of from 60 to 70 meters from the eye all objects, 



the most remote, appear equally distinct. The shorter the distance, the 

 >rter the line becomes; that is, during the highest degree of accommodation for 

 Sltuated a short distance behind the fixed point will appear 



9- The question whether it is possible for the lens to change its form partially, 



