STRUCTURE OF THE EYE. 



coinciding with, the axis of the crystalline lens, and through 

 the centre of the eye-ball, is called the optical axis, or the axis 

 of the eye. 



At a point of the posterior surface of the eye-ball between 

 the optical axis M T and the nose, the sclerotic coat is formed 

 into a tube, which leads backwards and upwards to the brain. 

 This tube contains within it the optic nerve, which at the point 

 c E, where it enters the eye-ball, spreads out over the inner 

 surface of the choroid and forms the retina, and includes the 

 hyaloid capsule containing the vitreous humour. 



The retina must therefore be regarded as nothing more than the 

 continuation and diffusion of the optic nerve. 



The retina, which in dissection admits of being easily separated 

 from the choroid, is absolutely transparent, so that the light or 

 colours which enter the inner chamber of the eye are not inter- 

 cepted by it, but penetrate it as they would any other thin and 

 perfectly transparent substance, and are only arrested by the black 

 coating spread upon the choroid. 



10. The following are the average numerical data connected 

 with the eye : 



lOOths ofaninch^ 



Radius of sclerotic coating . . . . 39 to 43 



Radius of cornea . 28 32 



External diameter of iris 



Diameter of pupil .... 



Thickness of cornea .... 



Distance of pupil from centre of cornea 



Distance of pupil from centre of crystalline 



Radius of anterior surface of crystalline 



Radius of posterior surface of crystalline . 



Diameter of crystalline 



Thickness of crystalline 



Length of optic axis .... 



43 47 

 12 28 



4 



8 



4 



28 39 

 20 24 

 39 

 30 

 87 95 



11. The limits of the play of the eye-baU are as follows : The 

 optic axis can turn in the horizontal plane through an angle of 

 60 towards the nose, and 90 outwards, giving an entire hori- 

 zontal play of 150. In the vertical direction it is capable of 

 turning through an angle of 50 upwards and 70 downwards, 

 giving a total vertical play of 120. 



12. When an image of any object is formed by a lens composed of 

 a single piece of glass or other transparent substance, it is always 

 tinged more or less at its edges with the prismatic colours, giving 

 it a sort of iridescence. This constituted a defect of the telescope, 

 which seemed so irremediable that many astronomers had recourse 

 by preference to reflectors, in which no such effect is produced. At 

 length it was discovered that this defect could be completely 



53 



