598 PHYSIOLOGY 



rays. A screen held at v will therefore show a bluish-violet 

 centre with a red margin ; at R the centre will be reddish and the 

 margin violet. 



In optical instruments this chromatic aberration is corrected by 

 combining glasses of different powers of dispersion, with the production 

 of so-called achromatic lenses. In the eye achromatism is practically 

 uncorrected. The difference in the focus of red and violet rays in the 

 eye amounts to about 0-5 mm, ; hence, if we are looking at a red and a 

 violet spot situated closely together, it requires a greater act of 

 accommodation to bring the image of the red spot on the retina than 

 is the case with the violet spot. The red spot therefore looks nearer, 

 i.e. more prominent. It may be this special effort of accommodation 

 necessary for the appreciation of red that makes this colour stand out, 

 so to speak, and be conspicuous as compared with the other colours of 

 the spectrum. 



The fact that as a rule we do not see coloured fringes around 

 every object that we look at is due, not to the optical, but to the 

 physiological qualities of the eye. When white light falls on the eye 

 and is focused by the latter on to the retina, it will be the rays ol 

 medium refrangibility which come to a point at the retina ; these 

 optically will be surrounded with a halo composed of the red and 

 violet rays, as is shown in the figure at /. The retina is, however, 

 comparatively insensitive for rays at the two extreme ends of the 

 spectrum. Moreover the strong illumination produced by the middle 

 rays of the spectrum ; i.e. about the yellow, at the centre of the 

 illuminated spot, by contrast depresses the excitability of the 

 surrounding parts of the retina, so that the halo due to the red and 

 violet rays is neglected and does not come into consciousness. 



ASTIGMATISM. We have assumed so far that the refracting 

 surfaces of the eye are practically spherical ; this does not apply 

 strictly either to the cornea or the lens. The small differences between 

 the curvatures of the cornea in the horizontal and vertical meridian 

 towards its centre do not as a rule give rise to appreciable disorders of 

 vision. In many cases, however, the asymmetry in the anterior sur- 

 face of the cornea is sufficient to cause a considerable difference in 

 the refraction of rays in the different meridians, and this disturbance is 

 known as astigmatism. 



The curvature of the vertical meridian of the cornea is nearly 

 always somewhat greater than that of the horizontal meridian. When 

 this difference is sufficiently pronounced it becomes impossible for 

 a definite image of a point of light to be formed on the retina, since 

 the rays which diverge from the luminous point in the vertical plane 

 are brought to a focus sooner than those in a horizontal plane. Such 

 an eye will therefore possess two posterior foci, one for the vertical 



