THE SENSES 



795 



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 \ mm. in front of the red. Thus, in Fig. 295 

 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 

 violet and the red would be external. For this reason it is impossible 



FIG. 295. CHROMATIC ABERRATION. 



The violet rays are brought to a focus V nearer 

 the lens than R, the focus of the red rays. 



FIG. 296. To SHOW DISPER- 

 SION IN EYE. 



View the figure from a distance 

 too small for accommodation. 

 Approach the eye towards it ; the 

 white rings appear bluish owing 

 to circles of dispersion falling on 

 them. A little closer, and the black 

 rings become white or yellowish- 

 white. 



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 accom- 

 modate 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. The dispersive power of the 

 eye, however, is so small, and the capacity of rapidly altering its 

 accommodation so great, that no practical inconvenience results from 

 the lack of achromatism, which, however, may be easily demonstrated 

 by looking at a pattern such as that in Fig. 296 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 

 curvature do not all lie in the same straight line ; (4) that the pupil 

 is eccentric, being situated not exactly opposite the middle of the 

 lens and cornea, but nearer the nasal side, and that in consequence 



