Chap, hi.] SIGHT. 841 



and the vitreous humour ; so that we have to deal with four sur- 

 faces, and, including the external air, four media. Indeed the 

 matter is in reality still more complicated, for the structure of 

 the lens, as we shall see, is such that the substance of the lens 

 differs somewhat in refractive power in different parts, the 

 central parts being more refractive than the peripheral parts ; 

 moreover the lens is covered in front by a capsule different in 

 structure from the lens itself. We may, however, neglect, with- 

 out fear of serious error, these smaller differences, and consider 

 the lens as one medium of uniform refractive power bounded by 

 an anterior and a posterior surface. The cornea again, as we 

 shall see, is not absolutely uniform in structure, but this we may 

 also neglect and consider the cornea as a medium, also of uni- 

 form refractive power, bounded by an anterior and a posterior 

 surface. Moreover, the posterior surface of the cornea is parallel 

 to (concentric with) the anterior surface or nearly so. Now 

 when the two surfaces which bound a medium are parallel to 

 each other we may, in dealing with refraction, neglect the thick- 

 ness of the medium entirely, we may suppose it to be absent and 

 treat the two surfaces as if they were one. We may therefore, 

 without serious error, neglect the substance of the cornea, and 

 consider the cornea as affording one surface, its anterior surface, 

 bounding the air in front from the aqueous humour behind. 

 Lastly, the aqueous humour differs in refractive power so little 

 from the vitreous humour that we may consider the two as form- 

 ing one medium. 



We have therefore to deal with three surfaces separating 

 three media, viz.: — first, the anterior surface of the cornea, at 

 which considerable refraction takes place as the rays of light 

 pass from the less refractive air into the more refractive aqueous 

 humour; secondly, the anterior surface of the lens, at which 

 again considerable refraction takes place as the rays pass from 

 the less refractive aqueous humour into the more refractive sub- 

 stance of the lens ; and lastly, the posterior surface of the lens, 

 at which refraction takes place as the rays pass from the more 

 refractive substance of the lens into the less refractive vitreous 

 humour. The three surfaces, differing in curvature, are all 

 approximately centred, symmetrically disposed around, the optic 

 axis of the system. This optic axis meets the retina, according 

 to some authorities, not quite at the part of the retina which, 

 under the name of fovea centralis, we shall hereafter speak of as 

 the centre of the retina, but a little above and to the nasal side 

 of that part; other authorities, however, maintain that it does 

 cut the retina at the fovea centralis. 



§ 527. The eye, therefore, even with the simplifications 

 which we have introduced, presents a much more complex op- 

 tical system than the one described above. It has, however, been 

 shewn mathematically that a complex optical system consisting 



