538 



PHYSIOLOGY 



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 close 

 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 of 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 surface 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 meridian, the other for the horizontal meridian. The 

 manner in which such rays diverge is shown in Fig. 266. 



