£52 



OPTICS. 



still darker, the posterior surface of this membrane 

 is covered all over with a black mucus, called the 

 pigmentum nigrum. 



From the part of the choroides, called the ciliary 

 circle, arise a set of radial fibres, turning inwards 

 towards the centre of the eye, and filled up be- 

 tween with a black mucus, giving it the appearance 

 of a membrane, as C a, D b. This serves to sup- 

 port the crystalline humour, to be described after- 

 wards ; and is called the ligamentum ciliare, or 

 ciliary ligament. 



The third and last membrane of the eye is called 

 the retina. This is a fine and delicate membrane, 

 being an expansion of the medullary part of the 

 optic nerve. It is spread like a net all over the 

 concave surface of the choroides, and terminates 

 at the ciliary ligament. It serves to receive the 

 images of objects produced by the refraction of 

 the different humours of the eye, and painted, as 

 it were, upon its surface. It is itself transparent, 

 but appears black, by reason of the pigmentum 

 nigrum spread underneath it. From the hinder 

 part of the eye (but not from the central part), 

 proceeds the optic nerve A, which conveys to the 

 brain the sensation produced upon the retina. 



The coats of the eye which invest and support 

 each other after the manner of the concentric coats 

 of an onion, or other bulbous root, inclose three 

 transparent bodies, called the aqueous, crystalline, 

 and vitreous humours. 



The aqueous humour is the most fluid, being 

 thin and clear like water : it fills up the space be- 

 tween the cornea and ciliary ligament, being di- 

 vided into two portions by the iris, which swims in 

 it. These are called the anterior and posterior 

 portions of the aqueous humour. 



