826 THE EYE AND VISION [CH. LV11I. 



cause. On diminution or cessation of the action of the ciliary 

 muscle, the lens returns to its former shape, by virtue of its elas- 

 ticity (fig. 527). From this it will appear that the eye is usually 



FIG. 527. Diagram representing by dotted lines the alteration in the shape of the lens on accommo- 

 dation for near objects. (E. Landolt.) 



focussed for distant objects. In viewing near objects the ciliary 

 muscle contracts ; the ciliary muscle relaxes on withdrawal of the 

 attention from near to distant objects. 



It is possible to calculate the curvature of the lens or cornea in the body, by 

 measuring the size of the image of an object upon it The radius (r) of curvature 



of a convex reflecting surface is given by the formula r= ; a is the distance of 



c 



the object from the surface, b the diameter of the image, and c that of the object. 

 a and c are easily measured ; b is measured by Helmholtz's ophthalmometer, the 

 principle of which is as follows : If a line is looked at through a plate of glass 

 placed obliquely between it and the eye, the line is shifted sideways to either right 

 or left ; if the glass plate is then placed obliquely at right angles to its previous 

 position, the line is shifted in the opposite direction. In the ophthalmometer there 

 are two glass plates intersecting each other at an angle; the image of a bright 

 horizontal line upon the lens or cornea is looked at through the junction between 

 the two plates ; one plate shifts the image to the right, the other to the left ; the 

 angle between the two plates is altered until the line appears as two distinct lines 

 just touching each other. The amount of shifting of each, which must therefore be 

 half the length of the image of the line, can be easily calculated if the thickness of 

 the glass plates, their refractive index, and the angle between them are known. 

 Double this result gives the size of the image on the surface under investigation. 



Range of Distinct Vision. Near -point. In every eye there is a 

 limit to the power of accommodation. If a book be brought nearer 

 and nearer to the eye, the type at last becomes indistinct, and cannot 

 be brought into focus by any effort of accommodation, however 

 strong. This, which is termed the near-point, can be determined by 

 the following experiment (Scheiner). Two small holes are pricked in 

 a card with a pin not more than a twelfth of an inch (2 mm.) apart ; 

 at any rate their distance from each other must not exceed the 

 diameter of the pupil. The card is held close in front of the eye, 



