646 



THE SENSES 



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 

 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 



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



measured. It is usually about five or six inches, 12 to 15 cm. In the accom- 

 panying figure, 461, the lensfr represents the eye; ,/, 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 be brought nearer 



FIG. 462. Diagram of the Axes of Rotation of the Eye. The thin lines indicate axes of 

 rotation, the thick the position of muscular attachment. 



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 images h, g, are 

 formed, one from each hole. It is interesting to note that when in this way 



