3 8 THE ART OF PROJECTING. 



THE MEGASCOPE. 



Photographs that are taken especially for projection 

 with the magic lantern are often called transparencies 

 because all of the lighter parts of the pictures are made 

 as transparent as possible, and they are shown by light 

 that is transmitted through them. If one would ex- 

 hibit a picture like a stereoscopic view or a common 

 carte de visite, it is evident that recourse must be had to 

 some other arrangement. The light must be reflected 

 from the picture, but when only the ordinary amount 

 which is reflected from a surface of nine square inches 

 is distributed over seventy-five or a hundred square feet, 

 it is evident that it will be but dimly visible. If a large 

 amount of light is concentrated upon the picture it will, 

 of course, reflect more, and its image will be corres- 

 pondingly brighter. This can be effected in two ways : 

 first, by using a large lens, or second, by using a large 

 concave mirror. 



The following figures will serve to show how this may 



Fig. 22. 



be done. When sunlight is used, the larger the con- 

 denser the better. One seven or eight inches in diam- 

 eter, if possible, should concentrate the light upon a 



