THE SENSES 799 



the near point is farther from the eye. The defect is cor- 

 rected by convex glasses (Fig. 299). Hypermetropia, unlike 

 myopia, is present at birth. 



Presbyopia, or the long-sightedness of old age, is not to 

 be confounded with hypermetropia. It is essentially due 

 to failure in the power of accommodation, chiefly through 

 weakness of the ciliary muscle, but partly owing to increased 

 rigidity and loss of elasticity of the lens. Images of distant 

 objects are still formed on the retina of the unaccommodated 

 eye with perfect sharpness ; i.e., the far point of vision is not 

 affected. But the eye is unable to accommodate sufficiently 

 for the rays diverging from an object at the ordinary near 

 point ; in other words, the near point is farther away than 

 normal. Convex glasses are again the remedy. 



The near point of distinct vision can be fixed in various 

 ways among others, by means of Scheiner's experiment 

 (Practical Exercises, p. 858). Two pin-holes are pricked in 

 a card at a distance less than the diameter of the pupil. 

 A needle viewed through the holes appears single when it is 

 accommodated for, double if it is out of focus. The near 

 point of vision is the nearest point at which the needle 

 can still, by the strongest effort of accommodation, be seen 

 single. 



Astigmatism. It has been mentioned that slight differences 

 of curvature along different meridians of the refracting 

 surfaces exist in all eyes. But in some cases the difference 

 in two meridians at right angles to each other is so great as 

 to amount to a serious defect of vision. To this condition 

 the name of ' astigmatism ' or ' regular astigmatism ' has 

 been given. It is usually due to an excess of curvature in 

 the vertical meridians of the cornea, less frequently in the 

 horizontal meridians ; occasionally the defect is in the lens. 

 Rays proceeding from a point are not focussed in a point, 

 but along two lines, a horizontal and a vertical, the hori- 

 zontal linear focus being in front of the other when the 

 vertical curvature is too great, behind it when the horizontal 

 curvature is excessive. The two limbs of a cross or the two 

 hands of a clock when they are at right angles to each other 

 cannot be seen distinctly at the same time, although they 



