LONG AND SHORT SIGHT. 



convex glass is used, for short sight the double-concave (fig. 162). We 

 know the burning-glass gives us a small image of the sun as it converges 

 the rays to its focus. But lenses will do more than this, and in, the 

 PHOTOGRAPHIC CAMERA we find great interest and amusement. 



Fig. 158. Diverging rays. 



Photography (or writing by light) depends upon the property which 

 certain preparations possess of being blackened by exposure to light while 

 in contact with matter. By an achromatic arrangement of lenses the 



Fig 159. Hypermetropia (long sight) 



Fig. 1 60. Myopia (short sight). 



camera gives us a representation of the desired object. Fig. 163 shows the 

 image on the plate, and figs. 164 and 165 the arrangement of lenses. 



To Porta, the Neapolitan physician, whose name we have already men- 



Fig. 1 6 1. Concave and convex lenses. 



tionea more than once, is due the first idea of the Photographic Camera. 

 He found that if light was admitted through a small aperture, objects from 

 which rays reached the hole would be reflected on the wall like a picture. 



Fig. 162. Lenses for long and short sight. 



To this fact we are indebted for the CAMERA OBSCURA, which receives the 

 picture upon a plane surface by an arrangement of lenses. In fact, Porta 

 nearly arrived at the Daguerreotype process. He thought he could teach 



