732 



THE SENSES 



letters are black and distinct but just within this point the outlines present 

 a just perceptible indistinctness, a loss of sharpness of border. This limit 

 is termed the near -point of vision. The near-point can be determined 

 by the experiment of Scheiner. Two small holes not more than 2 mm. 

 apart are pricked in a card with a pin; at any rate their distance from 

 each other must not exceed the diameter of the pupil. The card is held 

 with the holes close in front of the eye, and a small needle viewed 

 through the pin-holes. At a moderate distance it can be clearly focused, 

 but when brought nearer, within a certain point, the image appears 

 double and more or less blurred. This point where the needle ceases 

 to appear single is the near-point of vision. Its distance from the eye 

 can of course be readily measured. It is usually about five or six inches, 

 12 to 15 cm. In the accompanying figure, 461, the lens b represents the 

 eve ; e > /> tne two pin-holes in the card; nn the retina; a represents 



FIG. 461. Diagram of Experiment to Ascertain the Minimum Distance of Distinct Vision. 



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

 the two pencils of light coming through the holes e and / are focused 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 the 

 same as if the retina were shifted forward to mm. Two indistinct images, 

 h, g, are formed by the converging pencils of light, one from each hole. 

 It is interesting to note that when in this way two shadowy images are 

 produced, the lower one, g, really appears out in space in the position Q, 

 while the upper one appears in the position P. This may be readily veri- 

 fied by covering the holes in succession. This is due to the fact that when 

 points on the retina outside the visual axis are stimulated the sensation is 

 referred to an object in space along the line of the secondary optic axis 

 that cuts the retina at the point stimulated. 



During accommodation two other changes take place in the eyes. The 

 two eyes converge by the action of the extra-ocular muscles, chiefly by the 

 internal and inferior recti or internal and superior recti. The pupils also 

 contract. 



