612 A TEXTBOOK OF PHYSIOLOGY 



during the upward movement, will be seen to have a border of blue, 

 while during the downward movement it has a reddish-yellow one. 



The refracting surfaces of the eye are not strictly spherical, but 

 of the forms known as ellipsoids of rotation surfaces formed by the 

 rotation of an ellipse upon one of its axes. The result of this is that 

 their curvatures vary in different planes. Usually, in most eyes, 

 the curve is more convex in the vertical meridian than in the hori- 

 zontal. Vertical rays are therefore, after passing through the eye, 

 brought to a focus nearer than horizontal rays. For this reason, 

 stars are seen " star-shaped." Were the eye absolutely correct, 

 stars would be luminous points. This defect is known as astigmatism. 

 When, however, the defect of vision is so great that, owing to 

 differences in refraction in different meridians, the subject is 

 incommoded seriously, there is present some greater irregularity of 

 the cornea or lens. It is necessary to correct the defect by 

 reinforcing the curvature of the weaker meridians by means of 

 cylindrical lens. 



SECTION III 



THE ADJUSTMENT OF THE EYE FOR DIFFERENT 

 DISTANCES 



WHEN at rest, the eye is normally adjusted to focus parallel 

 rays upon the retina. For practical purposes, rays coming from 

 a distance greater than 6 metres may be regarded as parallel. Inside 

 this distance, the rays to the normal eye become divergent, and for 

 a clear image to be formed on the retina it is necessary for them to 

 be more refracted, otherwise a blurred image is formed, and the object 

 is not distinctly seen. Thus, while standing 20 feet from a window, 

 a needle held 2 feet from the eye is blurred when the window-sashes 

 are distinct. Conversely, when the needle is seen clearly, the window- 

 sashes are blurred. In looking at the needle under these conditions, 

 we are conscious of the effort of accommodating the eye for near 

 vision. Normally, although we are not conscious of it, the effort 

 required helps us to form our judgment of the distance of 

 objects. The nearer the object, the greater the effort required to 

 accommodate, and the sooner fatigue is experienced. Accommodation 

 is brought about, not, as in the camera, by altering the position of 

 the sensitive plate, but by altering the refractive power of the lens. 

 When we accommodate for near vision, the anterior surface of the 

 lens becomes more convex. This can be shown by viewing in a dark- 

 ened room the images of a candle-flame reflected from the eye of a 

 subject. When the light is properly adjusted, three images are seen 

 one from the cornea, bright and erect ; one from the anterior surface of 

 the lens, apparently coming from near the centre of the pupil, feebler 

 than the first, but erect; and a third, more deep-seated, generally a 



