THE IMAGE-FORMING MECHANISM OF THE EYE 



655 



02 represent the counterclockwise angular displace- 

 ment of the principal meridians from the o to 180 

 degree meridian. 



Accommodation 



The refracting mechanism of the eye possesses the 

 ability of accommodating itself for different distances; 

 that is to say, the eye can focus on one object at a 

 given moment and on an object at a diflferent distance 

 a moment later. This represents a change in the re- 

 fractive state of the eye which is brought about by a 

 change in form of the lens and a slight movement for- 

 ward. The major part of this efTect is mediated by the 

 change in curvature of the front surface of the lens, 

 and it greatly simplifies our concept of how the eye 

 works if we assume that this is the only variable. If 

 as in the schematic eye the pupil is centered upon the 

 optical axis of the lens, chief rays through the center 

 of the pupil cross the axis at the front surface of the 

 lens and the refraction of such rays is not affected by 

 a change in curvature. There is therefore no change 

 in the ratio of 7' to 7. 



Static Refraction of the E\e 



When accommodation is relaxed, the point R for 

 which the eye is accommodated in a given meridian 

 is known as the far point Qmncliim remotum") for that 

 meridian. The reciprocal Qi/RS') of the distance from 

 the far point (R) to the spectacle point (5) is defined 

 as the static refraction of the eye and is measured in 

 diopters. The spectacle point corresponds to the back 

 surface of a spectacle lens and lies 14 mm in front of 

 the cornea. For all practical purposes it coincides 

 with the primary focal point. 



Emmetropia is the condition in which the far 

 point lies at infinity and in which the static refraction 

 equals zero. Ametropia is the condition in which the 

 far point does not lie at infinity but at some finite dis- 

 tance either in front of or behind the spectacle point. 

 Myopia is the positive t) pe of ametropia in which the 

 far point lies at some finite distance in front of the 

 spectacle point. Hyperopia is the negative type of 

 ametropia in which the far point lies behind the 

 spectacle point. 



When an eye is equally ametropic in all meridians, 

 it is said to have a spherical error of refraction which 

 may be either hyperopic or myopic. When the static 

 refraction differs in the various meridians, the eye is 

 said to have an astigmatic error of refraction. 



Correction for Ametropia 



That lens which will permit an ametrope to see 

 lines clearly in all meridians at 6 m with relaxed ac- 

 commodation is called the distance correction. The 

 myope needs a minus lens and the hyperope a plus 

 lens, as shown in figure 14. In each case the refracting 

 power of the lens is the same in all meridians and is 

 equal numerically to the static refraction but opposite 

 in sign. Myopia for example is a positive ametropia 

 which is neutralized with a minus lens. 



A person with astigmatism requires a lens which 

 varies in power from meridian to meridian. Such a 

 lens has a toric or cylindrical surface on one side and 

 a plane or spherical surface on the other and is 

 equivalent to a combination of a spherical lens with 

 a cylindrical lens. The cylindrical lens compensates 

 the astigmatic component of the refractive error, and 

 the spherical lens compensates the residual spherical 

 component. 



In designing a lens for a given person many com- 

 binations of surfaces may be used on the two sides 

 to provide the correction for the ametropia, and 

 other factors have to be considered in selecting the 

 particular curves to be used. The thickness and index 

 can also be varied although the glass normally used 

 has an index of 1.523 for sodium light. The lens may 

 be designed to compensate for its own aberrations, to 

 provide a specified amount of angular magnification 

 in addition to refracting power and to minimize re- 

 flections, and some consideration is always given to 

 breakage and weight on the face. Plastic lenses are 

 sometimes used instead of glass lenses. 



The ordinary ophthalmic lens is mounted with its 

 back surface at or near the spectacle point 14 mm from 

 the cornea and with its optic axis passing through 

 the center of rotation of the eye. 



A corneal contact lens is worn in contact with the 

 cornea, while the scleral type contact lens contacts 



FIG. 14. Spectacle lenses for the correction of spherical 

 ametropia. 



