VISION 



1027 



(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. 



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. In addition, the 

 anterior surfaces of the cornea and Jens 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 re- 

 fracting 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. Fig 423 .Spherical Aberration. Rays passing 



(2) Chromatic Aberra- through the more peripheral parts of a biconvex 

 tion. All the rays of the lens L && brought to a focus F nearer the lens than 

 spectrum do not travel F', the focus of rays passing through the central 

 with the same velocity portions of the lens, 

 through a lens, and are, 



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

 nearer the lens than the long red rays. It was at one time supposed that 

 this chromatic aberration, as it is called, is compensated in the eye; and 

 it is said that this mistake gave the first hint that Newton's dictum as 

 to the proportionality between deviation and dispersion was erroneous, 

 and led to the discovery of achromatic lenses. But in reality the eye 

 is not an achromatic combination; and the violet rays are focussed 

 about J mm. in front of the red. Thus, in Fig. 424 the white light 

 passing through the lens is broken up into its constituents: the violet 

 focus is at V, and the red at R, behind it. A screen placed at R would 

 show not a point image, but a central point surrounded by concentric 

 circles of the spectral colours, with violet outside. If the screen was 

 placed at V, the centre would be violet and the red would be external. 

 For this reason it is impossible to focus at the same time and with perfect 

 sharpness objects of different colours : a red light on a railway track 

 appears nearer than a blue light, partly perhaps for the reason that it is 

 necessary to accommodate more strongly for the red than for the blue, 

 and we associate stronger accommodation with shorter distance of the 

 object, although other data are also involved in such a visual judgment. 

 When we look at a white gas-flame through a cobalt glass, which allows 

 only red and violet to pass, we see either a red flame, surrounded by a 



