IfO 



BURNING GLASS; 



one side flat and the other convex or concave are less povi''* \ 

 erful in their refractions, than those which have been de- 1 

 scribed. They are called plano-convex and plano-concave. 

 The focus of the former is at the distance of the diameter ; 

 of a sphere, of which the convex surface of the lens forms ' 

 a portion. The last kind of lens is called a menis'cus, being j 

 convex on one side and concave on the other, like the glass i 

 or crystal of a watch. 



AH the parallel rays of the sun which pass through a , : 

 convex glass are collected in its focus, and the force of the . 

 heat there is to the common heat of the sun, as the surface | 

 of the glass is to the surface of the focus. If a lens four 

 inches in diameter collect the sun's rays into a focus at the J 

 distance of twelve inches, the image will not be more than ] 

 one tenth of an inch in diameter : the surface of this little < 

 circle is one thousand six hundred times less than the surface - 

 of the lens, and consequently the heat will be one thousand 1 

 six hundred times greater at the focus than at the lens. A ^ 

 globular decanter of water acts as a double convex lens, and I 

 furniture has been set on fire by leaving one incautiously j 

 exposed to the rays of the sun. A gentleman of London ] 

 formed a burning-glass three feet in diameter, and when i 

 fixed in its frame, it exposed a clear surface of more than ; 

 two feet eight inches in diameter, and its focus, by means ^ 

 of another lens, was reduced to a diameter of half an inch. \ 

 The heat produced by this was so great that iron plates were J 

 melted in a few seconds ; tiles and slates became red-hot in ^^ 

 a moment, and were vitrified, or changed into glass ; sulphur, ; 

 pitch, and other resinous bodies, were melted under water ; I 

 gold was rendered fluid in a few seconds- But notwithstand- 1 

 ing this intense heat at the focus, the finger might, without 

 the smallest injury, be placed in the cone of rays within an ': 

 inch of the focus. On bringing the finger nearer, a sensa- *j 

 tion was felt like that produced by a sharp lancet, and not ] 

 at all similar to the pain occasioned by the heat of fire or a * 

 candle. Substances of a white colour were difficult to be 1 

 acted upon. Pure water in a clear glass decanter will not i 

 be warmed by the most powerful lens, but a piece of wood * 

 placed in the water may be burned to a coal. If a cavity ; 

 be made in a piece of charcoal, and the substance to be 

 acted on be put in it, the effect produced by the lens will ; 

 be much increased. Any metal thus enclosed melts in a j 



