8 FORMATION OF RETINAL IMAGES. [BOOK in. 



by several surfaces, the latter differing from each other in curvature, 

 though being approximately spherical. Rays of light in passing 

 from an external object to the retina traverse in succession the 

 following surfaces and media : tin- anterior surface of the cornea, 

 the substance of the cornea, the posterior surface of the cornea, 

 the aqueous humour, the anterior surface of the lens, the substance. 

 of the lens, the posterior surface of the lens, and the vitreous 

 humour; so that we have to deal with four surfaces, 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, without 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 uniform 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 thickness 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 

 ] lower so little from the vitreous humour that we may consider 

 the two as forming 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 consider- 

 able refraction takes place as the rays pass from the less refractive 

 aqueous humour into the more refractive substance 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 



