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V. On the Construction of Polyzonal Lenses, and their Combination 

 with plain Mirrors, for the purposes of Illumination in Light- 

 Houses. By DAVID BREWSTER, LL. D. F. R. S. Lond. & 

 SEC. R S. Edin. 



(Read May 7. 1827J 



IN the year 1811, when I was occupied in drawing up an ar- 

 ticle on Burning Instruments for the Edinburgh Encyclopaedia, 

 my attention was in a particular manner directed to the con- 

 struction of Large Lenses, and to the different methods by which 

 they could be combined with plane and spherical mirrors, for 

 the purpose of obtaining an intense heat from the concentration 

 of the solar rays. I was thus led to examine the inventions and 

 contrivances which had been previously proposed by others, for 

 accomplishing the same object ; and after giving a historical ac- 

 count of them, I proceeded to describe the improvements and 

 constructions which had occurred to myself. 



In this inquiry, my attention was particularly arrested by an 

 ingenious speculation of the celebrated BUFFON, for augmenting 

 the power of Burning Lenses, by grinding out a portion of the 

 glass, and thus diminishing their thickness, without altering 

 their focal length. This idea will be understood by referring to 

 Plate III. Fig. 1., which is BUFFON'S own perspective representa- 

 tion of it, and which he has described in the following words : 



" This method consists in working my piece of glass by steps. 

 To make myself better understood, let us suppose that I wish 



VOL. XI. PART I. E 



