GIL LVI.] REFRACTION OF LIGHT 777 



black pigment, the screen by the layer of rods and cones of the retina, 

 and the lens by the refracting media. In the case of the camera, 

 the screen is enabled to receive clear images of objects at different 

 distances, by an apparatus for focussing. The corresponding con- 

 trivance in the eye is called accommodation. 



The iris, which allows ni'ore or less light to pass into the eye, 

 corresponds with the diaphragms used in the photographic apparatus. 



The refractive media are the cornea, aqueous humour, crystalline 

 lens, and vitreous humour. The most refraction or bending of the 

 rays of light occurs where they pass from the air into the cornea ; they 

 are again bent slightly in passing through the lens. Alterations in 

 the anterior curvature of the lens lead to accommodation. 



We may first consider the refraction through a transparent 

 spherical surface, separating two media of different density. 



The rays of light which fall upon the surface exactly perpendicu- 

 larly do not suffer refraction, but pass through, cutting the optic 

 axis (0 A, fig. 578), a line which passes exactly through the centre 



FIG. 578. Diagram of a simple optical system (after M. Foster). The curved surface, b, d, is supposed 

 to separate a less refractive medium towards the left from a more refractive medium towards the 

 right. 



of the surface, at a certain point, the nodal point (fig. 578, N), or 

 centre of curvature. Any rays which do not so strike the curved 

 surface are refracted towards the optic axis. Kays which impinge 

 upon the spherical surface parallel to the optic axis, will meet at a 

 point behind, upon the said axis which is called the chief posterior 

 focus (tig. 578, F x ); and again there is a point on the optic axis in 

 front of the surface, rays of light from which so strike the surface 

 that they are refracted in a line parallel with the axis d f'\ this 

 point (fig. 578, F 2 ) is called the chief anterior focus. The optic axis 

 cuts the surface at what is called the principal point. 



It is quite obvious that the eye is a much more complicated 

 optical apparatus than the one described in the figure. It is, how- 

 ever, possible to reduce the refractive surfaces and media to a simpler 

 form when the refractive indices of the different media and 



