THE EYE, AND THE PHENOMENA OF VISION. 



347 



SECTION lY, 



THE EYE, AND THE PHENOMENA OF VISION. 



If an opening 

 be made in the 

 Bide of a dark 

 chamber how 

 will images of 

 external ob- 

 jects be repre- 

 aeated? 



707. If we make a small aperture through the shutter of a 

 darkened room, the images of external objects will be pic- 

 tured indistinctly, and in an inverted position, upon the op- 

 posite walL The reason of this will appear evident from aa 

 inspection of I'ig. 280. It will be seen that the rays of light 

 diverging from the top and bottom of the object cross each 

 other in passing through the aperture, and consequently form an inverted 

 image. This image is rendered more distinct with a small aperture than with 

 a large one, since, in the first case, the rays which proceed from any particu- 

 lar part of the object fall only upon the corresponding part of the image, and 

 are not scattered indiscriminately over tlie whole picture, as they would be 

 if the aperture was larger. 



Fig. 280. 



Describe the ^^' '^ ^'^^ place of the room ■\\-ith an aperture in the shutter, 



construction of We substitute a dark box, with a double-convex lens fitted 

 Obscura.''™^'^* into one side, a picture will be formed on the opposite side of 

 the box, or upon a screen placed at the focal distance of the 

 lens. This picture will represent, with great beauty and distinctness, whatever 

 is in front of the lens, all the objects having their proper relations of light and 

 shadow, and their proper colors. Such an apparatus is called a Camera. 

 Obsccr.\. i' 



Fig. 281 represents the ordinary construction of the camera obscur.o. It 

 consists of a wooden rectangular box, into wliich the rays of the light penetrato 

 through a convex lens placed at the termination of the tube B. These rays, 

 if unobstructed, will form an image upon the opposite side of the box 0, but 

 if they are received upon a mirror, M, inclined at an angle of 45°, their direc- 

 tion is changed, and the image will be formed upon a screen, or plate of 

 ground glass, N, placed at the top of the box. By placing upon this screen a 

 sheet of tracing paper, the outlines of the image may be readily copied. 



