PHYSIOLOGY OF VISION. 



165 



ges of external objects may be produced in a darkened 

 room, as an illustration of the manner in which the same 

 effect is produced in the eye. 



498. Let a room be darkened so as to exclude all the 

 light in every direction, except through a small aperture 

 in a window-shutter. The consequence will be, that the 

 images of external objects, as trees, houses, and men, 

 will be seen painted in the inverted position, on the oppo- 

 site wall, or on a screen of white paper held before the 

 aperture. 



499. Cause of the Inverted Image. The reason why 

 these images are inverted, is, that the rays of light pro- 

 ceeding from the extremities of the objects must converge 

 in order to pass through the small aperture, and conse- 

 quently they cross each other at that point, so that the 

 lowest portion of the object is the highest part of the 

 picture. All this will be readily understood by a bare 

 inspection of Fig. 103, which represents a monument 



Fig. 103. 



with the course of the rays from its extremities, crossing 

 each other at the aperture, and a picture of the same 

 inverted on the inside of the room. 



500. This little experiment, which almost any one can 

 try, forms a faint camera-obscura. The picture, however, 

 becomes brighter by enlarging the aperture, but at the 



How may a simple camera-obscura be formed? Why is the picture 

 onghter when the rays pass through a small aperture ? Why is the image 

 rendered indistinct when the aperture is enlarged ? 



