VISION 991 



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

 founded 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 Schemer's experiment (Practical 

 Exercises, p. 1060). Two pin-holes are pricked in a card at a dis- 

 tance 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 accommoda- 

 tion, 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 fre- 

 quently 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 horizontal 

 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 can be successively focussed. The condition 

 may be corrected by glasses which are segments of cylinders cut 

 parallel to the axis (Practical Exercises, p. 1062). 



The Ophthalmoscope. The pupil of the normal eye is dark, and 

 the interior of the eye invisible, without special means of illu- 

 minating it. But this is not because all the light that falls upon the 

 fundus is absorbed by the pigment of the choroid, for even the pupil 

 of an albino appears dark when the eye is covered by a piece of 

 black cloth with a hole in front of the pupil. The explanation is 

 as fallows : 



Let the rays from a luminous point, P, be focussed by the -lens, 

 L, at P' (Fig. 410). It is plain that rays proceeding from P 7 will 

 PXT+IV retrace the path of those from P and be focussed at P. 

 Now, the eye receives rays from all directions, and, when it is 



