254 THE EYE. 



CAMERA OBSC 



sloping mirror M up to a sheet of thin paper laid upon a 

 glass plate above, where the observer can, if he pleases, 

 make a tracing with his pencil of the picture they produce. 



Sometimes the inclosure to exclude light from the sides 

 consists of a darkened room. The apparatus, however, for 

 forming the image in such a case is substantially the same, 

 consisting of a lens to form the image, and a mirror to pro- 

 ject it where it is most convenient to place the screen. 

 Sometimes the entering rays are reflected in the mirror 

 first, and afterward passed through the lens. It is neces- 

 sary in all these cases that the room should be darkened 

 by means of shutters, or in some other way. The appara- 

 tus is usually fitted into an opening made in one of the 

 shutters, while the others are entirely closed. 



In order to avoid the inconvenience of darkening a large 

 room in this way, a small building, like a summer-house, is 

 sometimes devoted exclusively to the purpose of a camera 

 obscura ; this is often done in large public gardens or 

 pleasure-grounds. 



