322 



WELLS'S NATURAL PHILOSOPHY. 



On what prin- 

 ciple may con- 

 Vfx lenses be 

 used as burn- 

 ing-glasses ? 



held on the other. A bright ring of light will FiG. 261, 



be observed on the paper, diminishing or in- 

 creasing in size according to the distance of 

 the paper from the glass. If the former is held 

 in such a manner that the ring of hght is re- 

 duced to a dazzling luminous point, as is rep- 

 resented in Fig. 262, it is then situated in tho 

 focus of the glass. 



680. From their prop- 

 erty of converging par- 

 allel rays to a focus, 

 convex lenses, like con- 

 cave mirrors, may be used for the 



Fig. 262. production of high temperatures, by con- 



centrating the rays of the sun. 



The ordinary burning, or sun-glass, as is represented 

 in Fig. 262, is simply a double-convex lens. By tho 

 employment of very large lenses, a degree of heat 

 may be produced far exceeding that of the best con- 

 structed furnace.* 



In the employment of convex lenses as 

 burning-glasses, the heat concentrated at the 

 focus is to the common heat of the sun, as 

 the area of the surface of the lens is to the 

 area of the focus. 



Thus, if a lens four inches in diameter collects the sun's rays into a focus at 

 the distance of twelve inches, tho focus will not be more than one tenth of an 

 inch in diameter; its surface, therefore, is 1,600 times less than the surface 

 of the lens, and consequently the heat will be 1,600 tunes greater at tho focus 

 than at the lens. 



681. The properties of a concave lens are greatly dif- 

 ferent from those of a convex lens. 



Rays falling upon a concave lens are so re- 

 fracted in passing through it, that they diverge 

 on emerging from the lens, as though they 

 issued from a focus behind it. The focus, 



now does the 

 heat at the fo- 

 cus of a burn- 

 ing-glass com- 

 Eare with the 

 eat of the sun f 



>V1iat is the 

 course of rays 

 falling upon a 

 double con- 

 cave lens ? 



* A lens of this character was constnicted many years since in England, three feet in 

 diameter, with a focal distance of si.'c feet eight inches. Exposed to the heat concentrated 

 in the focus of this powerful instrument, the metals were instantly melted, and even vol- 

 atilized, while quartz, flint, and the most refractory earthy substaDCos, were readily 

 liqueiled and caused to boil. 



