368 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



We must not omit to mention Berniere's burning glass, which was 

 constructed on the same principle as that of Tschirnhausen, and of 

 which Fig. 251 is a representation. The mechanism there shown was 

 for the purpose of enabling one man to arrange both lenses at the 

 same time, that the rays of the sun might always converge to the same 

 point. 



Burning-glasses have one disadvantage which renders them, inferior 

 to mirrors, namely, that in passing through a lens of any thickness 

 part of the heat rays are absorbed by the glass. This defect Buffon 



FIG. 251. Berniere's burning-glass. 



attempted to remedy by the intervention of polyzonal lenses, which 

 consist of a series of rings, each of which forms part of a lens of 

 constant focal distance, but of less thickness in the central portion. 

 We have already described these lenses, which have been brought to 

 perfection by Fresnel for the illumination of lighthouses. 



Burning-mirrors and lenses have been applied to the solution of 

 an interesting question in physical astronomy, whether the solar 

 rays which reach us by reflection from the moon have any sensible 

 heat. A great number of observers, from Lahire and Tschirnhausen 



