DIOPTEIC MECHANISM OF THE EYE 305 



comes into play renders the aberration rather less than it would 

 be with perfectly spherical curvature of the cornea. 



Under normal conditions spherical aberration is reduced by 

 the iris which checks the penetration of the more oblique incident 

 rays. Artificial limitation of the section of the luminous cone 

 that enters the eye, obtained by holding a card with pin-holes in 

 front of the eye, makes it possible as shown on p. 292 to read 

 printed characters, even when they are placed within the near 

 point of distinct vision. This is explained not only by the reduc- 

 tion of the diffusion-circles, but also by the correction of spherical 

 aberration. 



According to Gulls trand the difference of refraction in the 

 rays entering at the vertex of the cornea and at the margin of 

 the optic zone is quite four dioptres. This degree of spherical 

 aberration shows plainly that it depends not merely on the 

 cornea, but, to a certain extent, on the lens as well. 



(c) When the curvature of the visual 

 zone of the cornea is examined it is 

 found never to be really the segment of 

 a sphere, but rather a segment of an 

 unequal ellipsoid. Sections of the eye 

 along the vertical and the horizontal axes 

 do not give equal curves of intersection, 

 but the vertical meridian almost always 

 shows more pronounced curvature and 

 the horizontal less. 



Gullstrand (1896) invented a method 

 for the exact determination of the curva- FIG. IBS. piacido's keratoscope. 

 tures of the cornea in the different 



meridians. He photographed the mirror images of the cornea and 

 measured the photographic images under the microscope. For the 

 test-object he selected a figure with concentric circles, the so-called 

 keratoscope of Placido shown in Fig. 138, or quadrangular figures. 

 The deformation of the mirror images is the starting-point in 

 estimating the inequalities of the corneal curvatures. The 

 distance of the circles of the keratoscope and their distance from 

 the cornea being known, it is easy by measuring the deformations 

 of the circles in the different meridians to calculate the varying 

 asymmetry and irregularity of the corneal curvature. 



Fig. 139 is a diagram of the results obtained by Gullstrand" 

 with this method of measuring the horizontal and vertical 

 meridians of the eye. The two curves A and B show that in 

 the cornea investigated (apart from the little irregularities due 

 to the unequal distribution of the lachrymal fluid on the surface) 

 there was appreciable asymmetry and unequal curvature in the 

 horizontal and vertical meridians. The optic zone which corre- 

 sponds to the pupil, and which alone functions in vision, presents 



VOL. iv x 



