The Senses. 299 



Spherical aberration produces indistinctness of vision by 

 the production of circles of diffusion, caused by those rays 

 which met too early crossing each other and forming a 

 circle. 



Chromatic Aberration is due to the decomposition of white 

 light into its primary colours — viz., the formation of a 

 spectrum — by passing through a prism or a convex lens. 

 The colours of the spectrum are differently refracted, the 

 red being the least bent, the violet the most ; when there- 

 fore we can see the red distinctly the eye is not focussed 

 for the violet. Chromatic aberration is prevented in the 

 eye by the unequal refractive power of the various media. 



Astigmatism is due to irregularities in the curvatures 

 of the cornea. The curvature of the vertical meridian is 

 frequently greater than that of the horizontal meridian ; 

 by this difference in the curvatures of the refracting 

 surfaces whether lens or cornea, but generally the latter, 

 the rays of light cannot be focussed in one point, but in 

 two different focal points, of which the focus of rays falling 

 on the vertical meridian, lies in front of that for the rays of 

 a horizontal plane. 



Emmetropia. — We shall presently have to speak of accom- 

 modation, or the means by which the eye is focussed ; but 

 it is necessary here that we should understand that by the 

 term 'range of accommodation' is understood the power 

 the eye possesses of seeing distant objects 'as far as the 

 eye can reach,' and also of seeing near objects distinctly 

 up to 8 inches. Between 8 inches and infinity parallel 

 rays of light should still be focussed on the retina so as 

 to form a distinct image ; such an eye is termed emme- 

 tropic. 



But all eyes do not possess this range of vision ; some 

 are perfect for close objects, others are only perfect for 

 distant ones. The one eye is short-sighted, the other long- 

 sighted ; the explanation of these two conditions lies in the 

 shape of the eyeball. 



Myopia, or short-sightedness, is due to the eyeball being 

 too long, whereby the focal points of the image, instead of 



