012 



A MANUAL OF PHYSIOLOGY 



a fleeting dilatation of the pupil, distinct in cats, less marked in rabbits. 

 Subcutaneous injection has no effect. Instillation of the drug into 

 the conjunctival sac is without effect in the normal rabbit's eye, but 

 causes dilatation if the superior cervical ganglion has been removed. 



Functions of the Iris. In vision the iris performs two 

 chief functions : (i) It regulates the quantity of light allowed 

 to fall upon the retina. The larger the aperture of a lens, the 

 greater is its collecting power, the more light does it gather in 

 its focus. In the eye, the area of the pupil determines the 

 breadth of the pencil of light that falls upon 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 intol- 

 erance of bright 

 light ; and the same 

 is true in the con- 

 dition known as 

 irideremia, or con- 

 genital absence or 

 defect of the iris. 



(2) Another, and 

 perhaps equally im- 

 portant, 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. 



Defects of the Eye as an Optical Instrument. (i) Spherical Aberra- 

 tion. It is a property of a spherical refracting surface that rays of 

 light passing through the peripheral portions are more strongly 

 refracted than rays passing near the principal axis. Hence a 

 luminous point is not focussed accurately in a single point by a 

 spherical lens ; the image is surrounded by fainter circles of light, 

 the so-called circles of diffusion representing the rays which have 

 not yet come to a focus, or having been already focussed have 

 crossed and are now diverging. In the eye this spherical 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. InFaddition, the anterior 

 surfaces of the cornea and lens are not segments of spheres, but of 

 ellipsoids, so that the curvature diminishes somewhat with the dis- 

 tance from the optic axis, and, therefore, the refracting power as we 



FIG. 389. SPHERICAL ABERRATION. 



Rays passing through the more peripheral parts of 

 a biconvex lens L are brought to a focus F nearer the 

 lens than F', the focus of rays passing through the 

 central portions of the lens. 



