CH. x. 'CAMERA OBSCURA' AND MAGIC LANTERN. 73 



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 

 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) into the hole he 

 could get a still brighter and clearer picture at a particular 

 point in the room 



FIG. 7. 



Porta knew from the works of Alhazen that rays of light 

 are reflected in all directions from every object, and he ex- 

 plained 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.' You may perhaps have been into one 

 at the sea-side, where they build them for visitors to watch 

 the coloured reflection of the passers-by. In the camera 

 obscura, as it is now made, the glasses are so arranged that 

 the figures are upright. 



Porta saw at once how useful this invention would be 



