518 THE EYE AS OPTICAL INSTRUMENT. 



flattens. In short-sighted people the images of objects 

 are therefore formed not on the retina, but in front of 

 it; in long-sighted people objects at ordinary dis- 

 tances are seen indistinctly, because the rays of light 

 strike upon the retina before they have been brought 

 to a focus. The defect of short-sighted people is 

 amended by wearing spectacles with concave glasses, 

 which, combined with the too convex crystalline lens, 

 produce the effect of a less convex lens ; long-sighted 

 people, on the other hand, use spectacles with convex 

 glasses, which combined with the too flat crystalline 

 lens produce the effect of a more convex lens. 



If a person near a window turns his face to the latter 

 and holds a small mirror before one eye, so as to see the 

 reflected image of the eye, which at the same time re- 

 ceives a large quantity of direct light from the window, 

 the pupil of the eye will appear rather small, and the iris 

 will form a comparatively broad ring around it. If the 

 face is now directed towards the room and the eye 

 again observed in the mirror, the pupil will appear 

 much larger and the iris contracted to a narrow ring. 

 The iris thus takes the place of a self-regulating dia- 

 phragm, whose function it is to dilate the aperture and 

 admit more light when the light is weak, but to con- 

 tract the aperture and admit less light when the illu- 

 mination is too strong. If the quantity of light which 

 enters the eye were not capable of being regulated in 

 this manner, objects in faint light would fail to make a 

 sufficiently strong impression upon the retina, while, in 

 strong light, the retina would be excited so much as to 

 render all objects too dazzling for clear vision. 



When an object is seen, the eye is involuntarily 



