86 Dr BREWSTER on the Construction of Polyzonal Lenses, 



Perceiving, therefore, that a limit was necessarily set to the 

 construction of lenses of one piece, by the difficulty of procuring 

 colourless homogeneous glass, and by the trouble and expence of 

 casting and grinding it into its proper form, without flaws and 

 impurities, I conceived the idea of building a lens with a num- 

 ber of separate pieces, and, in 1811, 1 printed in the Edinburgh 

 Encyclopaedia the following method of carrying it into effect. 



" In order to remove these evils, and at the same time to di- 

 minish the expence, and simplify the construction of dioptric 

 burning instruments, the following construction has been pro- 

 posed by Dr BREWSTER. If it be required, for example, to con- 

 struct a burning lens 4 feet in diameter, it should be composed 

 of different pieces, as represented in Plate III. Fig. 2.,' where 

 ABCD is a lens of flint-glass, 18 inches in diameter. This lens 

 is surrounded by several segments, AGID, AGEB, BELC, 

 CLID, ground in the same tool with ABCD, but so formed with 

 respect to their thickness at AB and GE, &c. that they may ex- 

 actly resemble the corresponding portions of a solid lens. These 

 different thicknesses can be easily calculated, and there is no dif- 

 ficulty in giving the segments their proper form. This zone, 

 consisting of separate segments, is again surrounded with other 

 segments, GNOF, FOEP, PEMQ, QMLR, RLKS, SKIT, 

 TIHV, VHGN, each of which is six inches broad in the direc- 

 tion of the radius. The section of this lens is represented in 

 Fig. 3. where DE is the central portion, DC n, E o F the second 

 zone, and CA m, FB p the external zone. One of the segments 

 is shewn separately in Fig. 4. By this combination of segments, 

 a lens four feet in diameter will be formed, and will obviously 

 possess the same properties as if it consisted of solid glass. 

 The advantages of this construction may be very shortly enu- 

 merated. 



