CH. X. * CAMERA OBSCURA' AND MAGIC LANTERN. 75 



the room, opposite the hole, images of things outside the 

 window. These images were exactly the shape of the real 

 objects, and had always their proper colours ; as for example, 

 if a man was standing against a tree outside the house, the 

 green leaves of the tree and the different colours of the man's 

 clothes would be clearly shown on the wall. There was 

 only one peculiarity about the picture, it was always upside 

 down, so that the man stood on his head, or the tree with 

 its trunk in the air. The smaller the hole was, the clearer 



Fig. 7. 



were the outline and the colours of the image, and Porta 

 found that by putting a convex lens (that is, a glass with its 

 surfaces bulging in the centre, see p. 49) i^^^ ^^ ^^^^ ^^ 

 could get a still brighter and clearer picture at a particular 

 point in the room. 



Porta knew from the works of Alhazen that rays of 

 light are reflected in all directions from every object, and he 

 explained this image on the wall quite correctly, by saying 

 that the small hole lets in only one ray from each point of an 

 object outside; the other rays and those from the sky and 

 other objects being kept out by the shutter. Thus these 

 single rays fall directly on the wall without being mingled 

 with others, and so make a clear picture. It is easy to see 

 from fig. 7 that the image must be upside down, because the 

 rays cross in going through the hole. This simple discovery 

 of Porta's is called the ' Camera Obscura; or 'Dark Chamber.' 



