THE BOOK OF THE LANTEEN. 



17 



The best source of illumination for the lantern 

 is, when we exclude electricity for the reasons just 

 indicated, the lime-light. It has the good qualities 

 of intense whiteness, steadiness, ease of management, 

 portability, and although not a point, its area of radiance 

 is not much greater than the space covered by a pea. I 

 shall describe in detail the method of its production and 

 management later on. 



Having then a convenient form of intense light, we have 

 next to consider the means of using it to the best ad- 

 vantage. We must start with the acknowledgment that 

 in enlarging the image of a picture we must sacrifice a 

 large amount of light. But by using properly-constructed 

 lenses, we can make this loss as little as possible. As 

 already pointed out, the optical system of a lantern 

 consists of two distinct parts, the condenser and the 

 objective lenses. 



As many of my readers may be quite unacquainted with 



FIG. 10. 



the matter under consideration, I will point out why this 

 double system is necessary, and describe the work per- 

 formed by each set of lenses. In the annexed diagram, 

 fig. 10, L represents the lime cylinder, with rays of light 

 emanating from it and illuminating the picture P, which 



C 



