52 Dr BREWSTER on the Construction of Polyzonal Lenses 



II. On the Construction and Properties of the Polyzonal Lenses-. 



As the construction and properties of common lenses are well 

 known, I shall merely give a section of a common plano-convex 

 lens, and of a double convex lens, made of one solid piece of 

 glass, in order that they may be more readily compared with 

 the new lens shewn in Plate IV. 



Fig. 1. Is the section of a plano-convex lens of solid glass. 



Fig. 2. Represents a section of one of the new plano-convex 

 polyzonal lenses, in which the continuous surface is con- 

 vex. It consists of a single lens in the centre, surround- 

 ed with five zones, each of which zones is composed of se- 

 parate segments, as shewn in the plan, Fig. 7. 



Fig. 3. Represents a section of another plano-convex poly- 

 zonal lens, in which the continuous surface is plane. 



Fig. 4. Is the section of a double convex lens of solid glass. 



Fig. 5. Is the section of a double convex polyzonal lens. 



Fig. 6. Represents another form of the double convex poly- 

 zonal lens. 



Fig. 9. of Plate III. is a perspective view of a portion of the 

 five zones of a Double Convex Polyzonal Lens. 



Fig. 7. Represents a plan of the polyzonal lens, three feet 

 in diameter, in which the central lens is fourteen inches in 

 diameter *. 



In examining these figures, it will be seen, that the polyzo- 

 nal lenses differ from the common lens, in having, as it were, 



* A central lens of this size may be easily executed in flint-glass, free from any 

 considerable imperfections, for the late M. FEAUNHOFER undertook to execute a 

 flint lens for achromatic telescopes, eighteen inches in diameter ; and M. GUINAND 

 actually made one of that size. 



