THE SENSES 9,7 



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. 987). 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 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 hori- 

 zontal linear focus being 



in front of the other FlG . 395 . 



when the vertical curva- 

 ture 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 succes- 

 sively focussed. The condition may be corrected by glasses 

 which are segments of cylinders cut parallel to the axis 

 (Practical Exercises, p. 989). 



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

 and the interior of the eye invisible, without special means of 

 illuminating 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 follows : 



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

 L, at P 7 (Fig. 395). It is plain that rays proceeding from P' 

 will exactly retrace the path of those from P and be focussed at 



