CHAPTER ONE 



Introduction and Methods^^ , 



ANATOMY OF THE EYE 



THE DIOPTRIC APPARATUS 



For an organism to have a sense of sight, as distinct from a mere 

 awareness of light, it must possess a means of forming an image of 

 the external scene on its light-sensitive cells. This problem has been 

 solved by Nature in more than one way but, for the present purpose, 

 it is sufficient to describe briefly the main features of a vertebrate eye. 



In Fig. 1.1 is shown a horizontal section of the human eye. The 

 eyeball, an approximate sphere of 1 in. diameter, has a tough 

 opaque outer tunic called the sclera. In life only a portion of this 

 is visible (the white of the eye) but it actually covers the whole 

 surface except for a circular transparent portion, called the cornea, 

 through which light can enter. The cornea (0-5 mm thick) is of 

 rather greater curvature than the sclera, and has a refractive index 

 of 1-38. 



The space behind the cornea, known as the anterior chamber, is 

 filled with a transparent aqueous humour having a refractive index 

 similar to that of water (1-33). The cornea and aqueous humour 

 together are equivalent in refractive power to a lens of about 43 

 dioptres. (A 1 dioptre lens has a focal length of 1 metre, a 2 dioptre 

 lens, one of 0-5 metre, and so on.) 



Behind the anterior chamber is a transparent, double convex body 

 called the crystaUine lens. This is composed of concentric layers of 

 tissue, the curvature, hardness and refractive index of which increase 

 towards the centre, the refractive index increasing from 1-38 to 1-41. 



The space behind the lens is filled with the vitreous humour, a 

 transparent jelly with the same refractive index as the aqueous. In 

 contact with the vitreous humour is the retina, the sensitive layer 

 upon which the visual image is formed. 



Between the retina and the sclera is the choroid, a soft, brown, 

 extremely vascular layer containing melanin pigment. The function 



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