en. LV.] REFRACTION. 755 



The Eye as an Optical Instrument. 



The eye may be compared to a photographic camera, and the 

 transparent media correspond to the photographic lens. In such 

 a camera images of external objects are thrown upon a ground- 

 glass screen at the back of a box, the interior of which is painted 

 black. In the eye, the camera is represented by the eyeball with 

 its 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 contrivance in the eye is called accommodation. 



The iris, which is capable of allowing more or less light to pass 

 into the eye, corresponds with the diaphragms used in the photo- 

 graphic apparatus. 



The refractive media are the cornea, aqueous humour, crystal- 

 line 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 

 crystalline lens. Alterations in the anterior curvature of the 

 crystalline lens lead to what we have already termed accommo- 

 dation that is, the power the eye has for adjusting itself to 

 objects at different distances. 



We may first consider the refraction through a simple trans- 

 parent spherical surface, separating two media of different 

 density. 



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

 dicularly do not suffer refraction, but pass through, cutting the 

 optic axis (0 A, fig. 576), a line which passes exactly through the 

 centre of the surface, at a certain point, the nodal point (fig. 5 76, N), 

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

 curved surface are refracted towards the optic axis. Rays 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 (fig. 576, Fj) ; and again there is a point in 

 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. 576, F 2 ) is called the chief 

 anterior focus. The optic axis cuts the surface at what is called 

 the jyrincipal point. 



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

 optical apparatus than the ono described in the figure. It is, 



3 c 2 



