280 



NATURE'S TEACHINGS. 



The figure shows the double convex lens of the human eye 

 in its place. 



HAVING now seen something of the properties of the double 

 convex lens, we will examine its application to the Camera 

 Obscura. 



The lens is placed on one side of the camera, and is so made 

 that it can be slid backwards and forwards, and the focus 

 altered at will. The camera itself is a box completely closed, 

 so that no light can enter it except that which passes 

 through the lens. The latter is so arranged that the rays 

 which pass through it are crossed, and throw their image on 



EYE AND IMAGE. 



CAMERA OBSCURA AND IMAGE. 



the opposite side of the camera. In the photographic camera 

 a piece of ground glass is placed at the end, so that the rays 

 fall upon it, and the operator can see whether the image is a 

 good one. Of course the figures are reversed, so that the sitter 

 seems to be on his head, but that is a matter of no conse- 

 quence. Exactly the same effect is produced by the marine 

 telescope. 



The general structure of the camera is shown in the illustra- 

 tion, all needless details being omitted. 



I may here remark that the term " camera obscura," or 

 dark chamber, alludes to the fact that the box is com- 

 pletely closed, and, but for the rays which pass through the 

 lens, would be absolutely dark. 



THE opposite illustration shows the most perfect camera 

 obscura that can be imagined, namely, the human eye. Here 

 we have a dark chamber, a double convex lens, and an image 

 falling upon the back. Here the optic nerve comes into play, 

 takes cognisance of the image, and conveys the idea to the 



