ABNORMALITIES IN THE REFRACTION OF THE EYE 859 



instruments of this kind the luminous object, or target, is represented by a double 

 figure possessing a sharp mathematical outline, which in turn is doubled by a prism. 

 The four images thus obtained are first properly adjusted for a normal cornea. 

 When transferred upon an abnormally curved cornea, this defect is made apparent 

 immediately by their displacement toward one another. 



The condition of astigmatism may be corrected in one of two ways : 

 namely (a) by diminishing the refraction along the meridian of greatest 

 curvature or (6) by increasing the refraction along the meridian of 

 least curvature. Cylindrical lenses are used for this purpose, the 

 refracting power of which compensates precisely for the unequal cur- 

 vature of the cornea. In the former case we employ a lens designated 

 as minus and, in the latter, one designated as plus. 



Myopia. The condition of myopia or near-sightedness is due either 

 to an increase in the longitudinal diameter of the eyeball, or to an 

 excessive refracting power of the lens and other media of the eye. In 

 most instances, however, it is attributable to the former cause. 



The increase in the length of the eyeball may amount to a fraction 

 of a millimeter or to as much as 3.8 mm. Already with a lengthening 



FIG. 459. DIAGRAM TO ILLUSTRATE THE REFRACTION IN A MYOPIC EYE. 

 L, Luminous point focalized in L l in the vitreous humor. A concave lens L renders 

 these rays more divergent so that they are made to intersect upon the retina in L 2 . 



of 0.16 mm. the far point is moved to within 200 cm. from the eye, and 

 with an increase of 3.8 mm. to within 10 cm. The near point is at 

 this time only 5 to 6 cm. distant. Far objects, therefore, cannot be 

 brought to a focus upon the retina, unless the eye is equipped with an 

 artificial lens which exactly compensates for this defect. Thus, 

 parallel rays emerging from so short a distance as 6 m., actually inter- 

 sect in the vitreous humor in front of the retina. Distally to this 

 point of intersection, the rays again diverge and strike the retina widely 

 apart as a dispersion circle. It must be evident that this condition 

 cannot be improved by accommodating more sharply, because any 

 increase in the convexity of the lens must move the focal point farther 

 toward the lens, and give rise to an even greater dispersion of the 

 retinal image. Quite similarly, it may be reasoned that an object 

 held very close to the eye, is in a much better position, because its 



