516 THE EYE AS OPTICAL INSTRUMENT. 



parent and strongly refractive. The crystalline lens 

 consists of a fibrous, firm, and highly elastic substance. 

 The case of the eyeball is kept in shape by what are 

 termed the c humours,' watery or semi-fluid substances, 

 one of which, the aqueous humour r is hardly more than 

 water holding a few organic and saline substances in 

 solution ; it fills the corneal chamber, while the other, 

 the vitreous humour, is rather a delicate jelly than a 

 regular fluid, and keeps the sclerotic chamber full, that 

 is the larger space between the crystalline lens and the 

 retina. 



The crystalline lens, together with the concavo-con- 

 vex space in front of it filled with the aqueous humour, 

 constitutes a compound converging lens of small focal 

 length, which forms of an object in front of the pupil an 

 inverted real image smaller than the object, and pro- 

 jects the image upon the retina precisely as the lens of 

 a camera throws the image upon the ground-glass plate. 

 The network of nervous filaments of which the retina 

 consists is capable of being affected by external agents 

 in such a manner as to give rise to the sensation of light. 

 It is impossible to enter here more fully into the 

 mysterious mode by which that sensation is brought to 

 our consciousness ; but so much may be stated, that we 

 have only in that case a distinct vision of any object 

 when the optical image of it produced by the crystalline 

 lens upon the retina is sharp and distinctly defined. 

 If the outlines of the image are blurred, as, for example, 

 in images produced by lenses and received on a screen 

 which is not properly placed, or in the images of a 

 camera obscura of which the tube with the lens is not 

 at the requisite distance from the object, the object 

 appears to the eye dim and indistinct. In the ordinary 





