THE SENSES. 209 



produced by some power from without; and there seems no reason to 

 doubt that this power is supplied by the ciliary muscle. It is sometimes 

 termed the tensor choroidece. As this name implies, from its attachment 

 (p. 206, Vol. II.), it is able to draw forward the choroid, and therefore 

 slackens the tension of the suspensory ligament of the lens which arises 

 from it. The lens is usually partly flattened by the action of the sus- 

 pensory ligament; and the ciliary muscle by diminishing the tension of 

 this ligament diminishes, to a proportional degree, the flattening of which 

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

 muscle, the lens returns, in a corresponding degree, to its former shape, 

 by virtue of the elasticity of its suspensory ligament (Fig. 377). From 

 this it will appear that the eye is usually focussed for distant objects. In 

 viewing near objects the pupil contracts, the opposite effect taking place 

 on withdrawal of the attention from near objects, and fixing it on those 

 distant. 



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 line 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 



FIG. 378. Diagram of experiment to ascertain the minimum distance of distinct vision. 



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. In the accompanying figure (Fig. 378) the lens b 

 represents the eye; ef the two pinholes 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 be brought nearer than the 

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

 the two pencils, they meet at a point behind the retina. The effect is 

 VOL. II. 14. 



