OPTICS. S4J 



tenth of an inch broad. The surface of this little 

 circle is 1600 times less than the surface of the 

 lens, and consequently, the sun's rays must be ten 

 times denser within that circle; it is not, therefore, 

 surprising, that it burns with a degree of ardour 

 and violence exceeding any culinary fire. 



The most considerable of these glasses are those 

 which were made by M. Tschirnhausen and Mr. 

 Parker. The diameter of that of Tschirnhausen 

 was three feet, the focus was formed at twelve 

 feet, and its diameter one inch and a half, and 

 weighed 160 pounds. To render the focus more 

 vivid, it was collected a second time by a lens 

 placed parallel to the first, and so situated, that the 

 diameter of the cone of rays formed by the first 

 lens was exactly equal to the diameter of the 

 second lens, so that it received all the rays ; the 

 focus was contracted to eight lines, and its force 

 was increased proportionally. 



The lens made by Mr. Parker was formed of 

 flint-glass three feet diameter, and when fixed in 

 its frame, exposed a clear surface of two feet eight 

 inches and a half in diameter; weight, 212 pounds; 

 focal length, six feet eight inches ; diameter of the 

 focus, one inch. A second lens was used, which 

 reduced the focus to half an inch. 



Some of the principal effects produced by these 

 glasses were as follows : 



1. Every kind of wood took fire in an instant, 

 whether hard or green, or soaked in water. 



2. Iron plates grew hot in a moment, and then 

 melted. 



3. Tiles, slates, and all manner of earth, became 

 red, and vitrified. 



4. Sulphur, pitch, and all resinous bodies, melted 

 under water. 



VOL. I. r 



