304 PHYSIOLOGY CHAP. 



when the eye is accommodated to infinity. According to Wolf 

 (1888) the distance between these two focal points (which indi- 

 cate the degree of chromatic aberration in the eye) is - 75 mm. 

 According to Einthoven the distance between the focal point of 

 line D (orange-yellow) and F (blue) is O27 mm. These values 

 all relate to the schematic eye. 



The diffusion -circle of a ray of red light has a diameter of 

 about O'l mm. when the pupil is moderately dilated and the eye 

 accommodated to violet light; the diffusion-circle for red and 

 violet light is about 0'05 mm. in diameter when the eye is 

 accommodated to rays of medium wave-length. 



The following experiment is a convincing demonstration of 

 the chromatic aberration of the eye. On looking at a distant 

 flame through a cobalt-blue glass, which mainly allows red and 

 blue rays to pass, the focal point of the red rays falls on the 

 surface of the retina, that of the blue rays in front of it. In 

 this case a red flame outlined with blue is seen. If, on the other 

 hand, the flame looked at is close by, the focal point of the blue 

 rays falls on the retina, and that of the red rays behind it. The 

 flame now appears blue, outlined in red. 



Under ordinary conditions we scarcely notice the chromatic 

 aberration in our eyes, proving that it is a very slight defect 

 which does not perceptibly disturb the sharp outlines of the 

 visual images. The iris, by reducing the section of the cone of 

 light that enters the eye, undoubtedly acts as a diaphragm and 

 diminishes chromatic aberration. 



(&) Another error common both to the eye and to lenses with 

 a spherical surface depends on the fact that the hoinocentric rays, 

 i.e. such as start from any given point, have, even w r hen 

 monochromatic, a different focus according as they are more or 

 less central or peripheral. The rays nearer to the optical axis, 

 or falling on the central part of the lens, are less refracted ; 

 the more eccentric rays, falling near the edge of the pupil, are 

 more refracted. So that the rays of the homogeneous luminous 

 cone which enter the eye, or spherical lenses in general, converge 

 not in & focal point, but in a, focal line. This is known as spherical 

 aberration. It can be avoided in artificial lenses by altering 

 their curvature so that it decreases gradually from the central 

 point to the edge of the lens. It was formerly believed that the 

 spherical aberration of the eye was partially compensated by the 

 fact that the cornea exhibits the highest degree of curvature at 

 its centre, and is somewhat flattened at the edge. But Auber 

 and Gullstrand showed that the optic zone of the cornea, i.e. that 

 which serves for vision when the pupils are of normal width, is 

 not less curved at the periphery than at the centre. Only when 

 the pupil is artificially dilated by atropine can it be assumed 

 that the somewhat flattened peripheral corneal zone which then 



