THE CAMERA OBSCUEA. 253 



ed. How it happens that we see things right side up 

 when the picture that is formed in the eye by which we 

 see them is upside down, is a mystery which greatly puz- 

 zles the philosophers. 



If an exact model of the eye were made of porcelain and 

 glass, with a little peep-hole upon one side, so that we could 

 look in, we should see in the interior of it, on the back side, 

 a most perfect and beautiful picture of any external scene 

 or object toward which the opening in front might be 

 turned. A great variety of optical instruments have been 

 invented by man which act on the same principle as the 

 eye. There is a lens to concentrate the rays, a screen to 

 receive the image, and an inclosure to exclude all other 

 light except what comes through the lens. There is also 

 often a mirror to reflect the image, so that the screen that 

 receives it may be placed where it may be most conven- 

 iently viewed. 



There is another advantage in the use of the mirror in 

 these cases, for, by reflection in it, the image may be thrown 

 upon a horizontal screen, and in that case it may be looked 

 at from the side that will bring it right side up. 



There are many ways by which these arrangements are 

 inclosed for the purpose of excluding the outside light; for, 

 in order to produce the full effect, it is necessary that all 

 light, except what comes from the objects to be viewed, 

 should be excluded. Indeed, these instruments all take 

 their name from the Latin words meaning dark chamber, 

 or, rather, chamber dark, which words are camera obscura. 



The following engraving shows one of the forms in which 

 the camera obscura is often made. The rays R, which enter 

 the tube B, are made to converge that is, all which come 

 from any one point in the object are made to converge, 

 and they would fall upon the back of the box O, and form 

 an image there, were it not that they are reflected" by the 



