CH. LVI.] RANGE OF VISION 783 



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 



FIG. 585. Diagram representing by dotted lines the alteration in the shape of the leus on accommo- 

 dation for near objects. (E. Landolt.) 



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, 

 and a small needle viewed through the pin-holes. At a moderate 

 distance it can be clearly focussed, but when brought nearer, beyond 

 a certain point, the image appears double, or at any rate blurred. 

 This point where the needle ceases to appear single is the near-point. 

 Its distance from the eye can of course be readily measured. It is 

 usually about 5 or 6 inches (13 cm.). In the accompanying figure 

 (fig. 586) the lens "b represents the refractive apparatus of the eye; 

 e and / the two pin-holes in the card, nn the retina ; a represents the 

 position of the needle. When the needle is at a moderate distance, 

 the two pencils of light coming from e and /are focussed at a single 

 point on the retina nn. If the needle is brought nearer than the 

 near-point, the strongest effort of accommodation is not sufficient to 



