vi DIOPTRIC MECHANISM OF THE EYE 303 



the lens is absent. But the observations on which this statement 

 are founded can be explained without assuming any power of 

 accommodation. We pointed out that it is not indispensable to 

 distinct vision that the posterior focal point should coincide 

 exactly with the most external surface of the retina ; it is 

 enough if it falls within the mosaic membrane of rods and cones 

 (the structure of which will be discussed below), which is about 

 O'OG mm. thick. Besides this we know that sight can be fairly 

 clear when it is due, not to focal points, but to small diffusion- 

 ciivlos. 



VII. Thus far we have considered the eye as a perfect 

 optical instrument, as though the surfaces of curvature of its 

 refractive media were quite spherical and perfectly centred, and 

 these media completely homogeneous, transparent, and achromatic. 

 More accurate observation of the eye, however, shows that from 

 the dioptric standpoint it presents a series of imperfections or 

 defects which are normally insignificant, but may under abnormal 

 circumstances become so important as to 

 interfere with vision. Taking these defects . > 

 separately 



(a) Ordinary lenses break up white light, 



owing to the unequal refractibility or wave- 



lengths of the different coloured rays of which 

 it is composed. This is known as chromatic 



, . . m , ,, i i. Fin. 137. Chromatic aberra- 



aberration. Take tor example a cone 01 tion in an ordinary lens. 

 parallel white rays falling on a convex lens 

 (Fig. 137). The red rays, which are least refracted, unite at point 

 r of the optical axis, while the violet rays, which are most re- 

 fracted, converge at point v. Between these two extreme foci lie 

 those of the intermediate colours, indigo, light blue, green, etc. 

 If a diaphragm is inserted at point v a small white luminous circle 

 bordered with a red line appears; if the diaphragm is intro- 

 duced at r, the circle will be white with a violet edge. In both, 

 the centre of the circles is white because a number of rays of 

 different colours, which combine into white light, intersect there. 

 The distance vr, comprised between the focal points of the 

 extreme rays, may be taken as the measure of chromatic 

 aberration. 



This defect in common lenses can be avoided by the use of 

 an achromatic lens, formed by the combination of two lenses 

 (positive and negative) made of two substances with dili'erent 

 refractive indices, so combined that they keep their power of 

 convergence, while all trace of chromatic aberration disappears. 



The eye has no achromatic system ; its dioptric apparatus has 

 different focal distances for rays of difl'ereut wave-lengths. The 

 focal point of the violet rays (Fraunhofer's H line) is nearer the 

 lens than the focal point of the red rays (Fraunhofer's B line), 



