THE SENSES. 599 



ing 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 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. 407) 

 the lens b represents the eye; ef 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 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 



FIG. 407. Diagram of experiment to ascertain the minimum distance of distinct vision. 



to note that when two images are produced, the lower one g really ap- 

 pears in the position Q, while the upper one appears in the position P. 

 This may be readily verified by covering the holes in succession. 



Course of a Ray of Light. With the help of the diagram, repre- 

 senting a vertical section of the eye from before backwards, the mode in 

 which, by means of the refracting media of the eye, an image of an 

 object of sight is thrown on the retina, may be rendered intelligible. 

 The rays of the cones of light emitted by the points A B, and every other 

 point of an object placed before the eye, are first refracted, that is, are 

 bent towards the axis of the cone, by the cornea c c, and the aqueous 

 humor contained between it and the lens. The rays of each cone are 

 again refracted and bent still more towards its central ray or axis by the 

 anterior surface of the lens E E; and again as they pass out through its 

 posterior surface into the less dense medium of the vitreous humor. For 

 a lens has the power of refracting and causing the convergence of the 

 rays of a cone of light, not only on their entrance from a rarer medium 

 into its anterior convex surface, but also at their exit from its posterior 

 convex surface into the rarer medium. 



