988 



THE SENSES 



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 accommoda- 

 tion 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. 406 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 the optic axis, or straight 

 line joining the centres of curvature of the lens and cornea, does not 

 coincide with the visual axis, or straight line joining the fovea centralis 

 with the centre of the pupil, which is also the straight line joining the 

 centre of the pupil and any point to which the eye is directed in vision. 

 The angle between the optic and visual axis is about 5 (Fig. 397). 



Fig. 405. Chromatic Aberration. The violet 

 rays are brought to a focus V nearer the lens 

 than R, the focus of the red rays. 



Fig. 406. To show Dispersion in 

 Eye (v. Bezold). 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 i.e., circles of light of 

 different colours due to the 

 decomposition 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. 



(5) Muscae volitantes, the curious bead- 

 like or fibrillar forms that so often flit 

 in the visual field when one is looking 

 through a microscope, are the token that 

 the refractive media of the eye are not 

 perfectly transparent at all parts ; they seem to be due to floating opacities 

 in the vitreous humour, probably the remains of the embryonic cells from 

 which the vitreous body was developed. (6) Lastly, it may be men- 

 tioned that slight irregularities in the curvature of the lens exist in all 

 eyes, so that a point of light, like a star or a distant street-lamp, is not 

 seen as a point, but as a point surrounded by rays (irregular astigma- 

 tism). In bringing this review of the imperfections of the dioptric 

 media of the normal eye to a close, it may be well to explain that what 

 are defects from the point of view of the student of pure optics are not 

 necessarily defects from the freer standpoint of the physiologist, who 

 surveys the mechanism of vision as a whole, the relations of its various 

 parts to one another and to the needs of the organism it has to serve, 

 the long series of developmental changes through which it has come 

 to be what it is, and the possibilities, so far as we can limit them, that 

 were open to evolution in the making of an eye. The optician may 

 perhaps assert, and with justice, that he could easily have made a better 

 lens than Nature has furnished, but the physiologist will not readily 

 admit that he could have made as good an eye. 



