79 4 A MANUAL OF PHYSIOLOGY 



the lens. If this area was invariable, the retina would either 

 be ' dark from excess of light ' in bright sunshine, or dark 

 from defect of light in dull weather or at dusk. In order 

 that the iris may act as an efficient diaphragm it must be 

 pigmented, and it is the pigment in it which gives the colour 

 to the normal eye. The vision of albinos, in whose eyes 

 this pigment is wanting, is often, though not invariably, 

 deficient in sharpness. There is always intolerance of bright 

 light ; and the same is true in the condition known as 

 irideremia, or congenital absence or defect of the iris. 



(2) Another, and perhaps equally important, function of 

 the iris is to cut off the more divergent rays of a pencil of 

 light falling upon the eye, and thus to increase the sharpness 

 of the image. This leads us to the consideration of certain 

 defects in the dioptric arrangements of the eye. 



Cefects of the Eye as an Optical Instrument, (i) Spherical 

 Aberration. It is a property of a spherical refracting surface that 



rays of light passing 

 through the peri- 

 pheral portions are 

 more strongly re- 

 fracted than rays 

 passing near the prin- 

 cipal axis. Hence a 

 luminous point is not 

 focussed accurately 

 in a single point by 

 a spherical lens ; the 

 FIG. 294. SPHERICAL ABERRATION. image is surrou nded 



Rays passing through the more peripheral parts of a ^ y c i r cles of diffusion, 

 biconvex lens L are brought to a focus F nearer the _' , 

 lens than F', the focus of rays passing through the central ln tn ^ eye tniS Spn< 

 portions of the lens. rical aberration is 



partly corrected by 



the interposition of the iris, which cuts off the more peripheral rays, 

 especially in accommodation for a near object, when they are most 

 divergent. In addition, the anterior surfaces of the cornea and lens 

 are not segments of spheres, but of ellipsoids, so that the curvature 

 diminishes somewhat with the distance from the optic axis, and, 

 therefore, the refracting power as we pass away from the axis does not 

 increase so rapidly as it would do if the surfaces were truly spherical. 

 Further, the refractive index of the peripheral parts of the lens is 

 less than that of its central portions. 



(2) Chromatic Aberration. All the rays of the spectrum do not 

 travel with the same velocity through a lens, and are, therefore, 

 unequally refracted by it, the short violet rays being focussed nearer 



