THE SENSES 



913 



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 

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

 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 mm. in front of the red. Thus, in Fig. 390 

 the white light passing through the lens is broken up into its con- 

 stituents : 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 



FIG. 590. CHROMATIC ABERRATION. 

 The violet rays are brought to a focus V 

 nearer the lens than R, the focus of the red 

 ravs. 



FIG. 391. To SHOWJDISPER- 

 SION IN EVE (v. BEZOLD). 

 View the figure from a dis- 

 tance too small for accommoda- 

 tion. Approach the eye to- 

 wards it ; the white rings appear 

 bluish owing to circles of dis- 

 persion falling on them i.e., 

 circles of light of different 

 colours due to the decomposi- 

 tion of white light into its 

 spectral constituents by the 

 media of the eye. A little 

 closer, and the black rings be- 

 come white or yellowish -white. 



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 violet ring, or a violet flame surrounded by 

 a red ring, according as we focus for the red or for the violet rays. 

 But the dispersive power of the eye is so small, and the capacity 

 of rapidly altering its accommodation so great, that no practical 

 inconvenience results from the lack of achromatism, which, how- 

 ever, may be easily demonstrated by looking at a pattern such as 

 that in Fig. 391 at a distance too small for exact accommodation. 



It is also reckoned among the optical imperfections of the eye 

 (3) that the curved surfaces of the cornea and lens do not form a 

 ' centred ' system that is to say, their apices and their centres of 



58 



