Fig. 31. 



SECTION III. On the Structure of the Eye and Optical Instruments. 



THE body of the eye is of a spheri- Fig. 30. 



cal form (fig. 30). It has two mem- 

 braneous coverings ; the external 



one, a a , is called the sclerotica : 



this has a projection in that part of 



the eye which is exposed to view, 



b b, which is called the cornea, 



because, when dried, it has nearly 



the consistence of very fine horn, 



and is sufficiently transparent for 



the light to obtain free passage 



through it. The second membrane 



which lines the cornea, and enve- 

 lopes the eye, is called the choroid, 



c c c : this has an opening in front 



just beneath the cornea, which forms 



the pupil, d d, through which the 



rays of light pass into the eye. The 



pupil is surrounded by a coloured 



border of fibres, called the iris, e e, 



which, by its motion, always pre- 

 serves the pupil of a circular form, 



whether it be expanded in the dark 



or contracted by a strong light. 

 (Fig. 31.) 



The construction of the eye is so admirable, that it is capable of adapt- 

 ing itself, more or less, to the circumstances in which it is placed. In a 

 faint light the pupil dilates so as to receive an additional quantity of rays ; 

 and in a strong light it contracts, in order to prevent the intensity of the 

 light from injuring the optic nerve. The eyes suffer pain, when from 

 darkness they suddenly come into a strong light ; for the pupil being 

 dilated, a quantity of rays rush in before it has time to contract ; and 

 when we go from a strong light into obscurity, \ve at first imagine 

 ourselves in total darkness ; for a sufficient number of rays cannot gain 

 admittance into the contracted pupil to enable us to distinguish objects ; 

 but in a few minutes it dilates, and we clearly perceive what was before 

 invisible. The choroid, c c, is imbued with a black liquor, which serves 

 to absorb all the rays that are irregularly reflected, and to convert the 

 body of the eye into a more perfect camera obscura. When the pupil is 

 expanded to its utmost extent, it is capable of admitting ten times the 

 quantity of light that it does when most contracted. In cats, and animals 

 which are said to see in the dark, the power of dilatation and contraction 

 of the pupil is still greater : it is computed that their pupils may receive 

 one hundred times more light at one time than at another. Within these 

 coverings of the eye-ball are contained three transparent substances, called 

 humours. The first occupies the space immediately behind the cornea, 

 and is called the aqueous humour, ff, from its liquidity and its resem- 

 blance to water. Beyond this is situated the crystalline humour, g g, 



