THE 1M.\C;E-F()R.ML\G mechanism OF THE EYE 



657 



Optic Axis 0/ the Eye 



It has been assumed in connection with the sche- 

 matic eye that the refracting surfaces are centered on 

 a common axis, but the extent to which this is true 

 in the case of an actual eye can be tested by observing 

 the Purkinje images of a point of light held close to 

 the observer's eye. The subject is then made to follow 

 a fixation target which is moved about until the 

 Purkinje images line up or at least until the third and 

 fourth ones line up. This puts the observer's eye on 

 the path of a ray of light which passes through the 

 lens normal to both surfaces, and the extent to which 

 the center of the pupil and the center of curvature of 

 the front surface of the cornea are displaced from the 

 optic axis of the lens can then be directly observed. 



In general the optic axis (in so far a such an axis 

 exists) coincides with the pupillary axis, so that the 

 optic axis deviates temporalward from the primary 

 line of sight about 5 degrees and also about 2 degrees 

 downward (86, p. 77). For the purpose of specifying 

 this angle, it may be assumed that the two lines inter- 

 sect at the center of the entrance-pupil. 



The optic axis does not necessarily coincide with 

 the anatomical axis of the eye which may be defined 

 as the line connecting the geometrical center of the 

 cornea (front pole) with the geometrical center of 

 the sclera (back pole). However, the optic axis 

 and the anatomical axis do approximately coincide 

 in the average eye. Theoretically the anatomical 

 axis of the eye should coincide with the line 

 normal to the cornea at its geometrical center. 

 This is called the geometrical axis of the cornea. 



Coiifigmalion of Front Surface of Cornea 



The central portion of the cornea is usually spherical 

 or toroidal. With a keratometer one can determine 

 whether the cornea is spherical or toroidal and, if it 

 is toroidal, the principal meridians can also be deter- 

 mined. Furthermore, one can measure the radius of 

 curvature in each of the principal meridians of a 

 toroidal cornea and in any meridian of a spherical 

 cornea. 



The central portion of the typical cornea which 

 may be regarded as spherical or toroidal covers a 

 region about 4 mm in diameter and outside of this 

 area the curvature gradually decreases as the limbus 

 is approached. The center of the optical portion does 

 not necessarily fall at the center of the cornea (79, 

 p. 311 ; 86, p. 68). 



The exact form of the peripheral portion of the 

 cornea can be investigated in a number of ways. One 



can view the profile of the cornea with a microscope 

 or photograph the profile. One can examine it point 

 by point with an ordinary keratometer by using a 

 variable point of fixation. It can also be viewed with 

 a keratoscope or photographed with a photokerato- 

 scope. In this technique a reflected image of a series 

 of concentric circles is used. In one of the later models 

 (50) the concentric rings are arranged on a spherical 

 surface concentric with the eye so that the reflected 

 images cover the entire cornea. Another method is 

 that of sprinkling powder on the cornea and then 

 taking a stereophotograph which can later be ana- 

 lyzed like an aerial map. One can also take a mold 

 of the cornea as in fitting contact lenses and then 

 studv the configuration of the mold. 



Measurement of Internal Refracting Surfaces 



One can measure the position of the margin of the 

 iris with respect to the cornea and assume that this 

 lies in contact with the front surface of the lens (79, 

 p. 19, 334). The ophthalmophakometer (86, p. 80) 

 and the Blix ophthalmometer (79, p. 326) make use 

 of specular reflections at the surfaces to locate the 

 positions of the vertices and centers of curvature of the 

 surfaces. It is also possible to photograph the Purkinje 

 images (i, 2, 6, 45, 49, 94) and to calculate the radii 

 of curvature of the reflecting surfaces from this kind 

 of data. 



Fincham (26, p. 38) used diffusely reflected light 

 to produce an optical section of the refracting surfaces, 

 and lay viewing with a microscope having a cali- 

 brated fore and aft movement he was able to measure 

 directly the apparent separations of the surfaces. 

 A similar arrangement can be used with a camera 

 replacing the microscope (26, p. 44). A projective 

 transformation of the photographic image gives a 

 cross section of the eye. This inethod has the ad- 

 vantage of showing not only the configurations of 

 the surfaces but also gives the internal structure of 

 the lens. 



The measureiTients made on an internal surface 

 appl\- to the apparent surface viewed through the 

 refracting surfaces lying in front of the surface in 

 question. The concept of a thick mirror is helpful in 

 this connection. The center of curvature of the 

 apparent surface is the image of the actual center of 

 curvature formed by the refracting surfaces lying 

 in front, and the vertex of the apparent surface is 

 also the image of the vertex of the actual surface. 



