THE SENSE OF SIGHT. 163 



may be removed and yet distinctly seen. If the object be brought nearer 

 to the eye than 200 feet the accommodative power must come into play : 

 the nearer the object the more energetic must be the contraction of the 

 ciliary muscle and the consequent increase in the convexity of the lens. At 

 a distance of five inches, however, the power of accommodation reaches its 

 maximum : this is termed the punctum proximum, and indicates the nearest 

 point at which an object may be seen distinctly. The distance between 

 these two points is the range of accommodation. 



Optical Defects. Astigmatism is a condition of the eye which 

 prevents vertical and horizontal lines from being focused at the same 

 time, and is due to a greater curvature of the cornea in one meridian 

 than another. 



Spherical aberration is a condition in which there is an indistinctness of 

 an image from the unequal refraction of the rays of light passing through 

 the circumference and the centre of the lens; it is corrected mainly by the 

 iris, which cuts off the marginal rays, and only transmits those passing 

 through the centre. 



Chromatic aberration is a condition in which the image is surrounded by 

 a colored margin, from the decomposition of the rays of light into their 

 elementary parts. 



Myopia, or short-sightedness, is caused by an abnormal increase in the 

 antero-posterior diameter of the eyeball, or by a subnormal refracting power 

 of the lens; it is generally due to the first cause; the lens being too far 

 removed from the retina, forms the image in front of it, and the perception 

 becomes dim and blurred. Concave glasses correct this defect, by prevent- 

 ing the rays from converging too soon. 



Hyper met ropia, or long-sightedness, is caused by a shortening of the 

 antero-posterior diameter, or by an excessive refractive power of the lens ; 

 the focus of the rays of light would, therefore, be behind the retina. Con- 

 vex glasses correct this defect, by converging the rays of light more anteri- 

 orly. 



Presbyopia is a loss of the power of accommodation of the eye to near 

 objects, and usually occurs between the ages of 40 and 60 ; it is remedied by 

 the use of convex glasses. 



The Iris. The iris plays the part of a diaphragm, and by means of its 

 central aperture the pupil regulates the quantity of light entering the 

 interior of the eye ; by preventing rays from passing through the margin of 

 the lens it diminishes spherical aberration. The size of the pupil depends 

 upon the relative degree of contraction of the circular and radiating fibres ; 



