PHYSIOLOGY OP VISION. 



169 



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. 



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 

 opposite wall, or on a screen of white paper held before 

 the aperture. 



Cause of the Inverted Image. The reason why these 

 images are inverted, is, that the rays of light proceeding 

 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. 102, which represents a monument, 



Fi<r. 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. 



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 7 Why is the picture 

 brighter when the rays pass through a small aperture 1 Why is the 

 image rendered indistinct when the aperture is enlarged ? 



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