THE EYE AS AN OPTICAL INSTRUMENT. 181 



The natural camera obscura of the eye (seen in a diagram- 

 matic section in Fig. 28) has its blackened chamber globular in- 

 stead of cubical, and made not of wood, but of a thick, strong, 

 white substance known as the sclerotic coat. It is this which 

 is partly seen between the eyelids as ' the white of the eye.' 

 This globular chamber is lined with a delicate coat of winding 

 blood-vessels covered inside by black pigment. But the apple 

 of the eye is not empty like the camera : it is filled with a 

 transparent jelly as clear as water. The lens of the camera 



obscura is represented, first, by a convex transparent window 

 like a pane of horn (the cornea), which is fixed in front of the 

 sclerotic like a watch glass in front of its metal case. This 

 union and its own firm texture make its position and its curva- 

 ture constant. But the glass lenses of the photographer are 

 not fixed ; they are moveable by means of a sliding tube which 

 can be adjusted by a screw (Fig. 27, r), so as to bring the objects 

 in front of the camera into focus. The nearer they are, the 

 farther the lens is pushed forward ; the farther off, the more it 



