54 Dr BREWSTER on the Construction of Polyzonal Lenses 



be quite easy to obtain 50 or 100 Ib. for a polyzonal one. It is not, 

 however, necessary that the lens be made of one kind of glass. 

 Let us suppose that we have six different kinds of glass, with 

 six different refractive powers, we have only to form the central 

 lens of the least refractive glass, and the other zones of the other 

 kinds of glass, so that the refractive power of the glass of any 

 one zone is greater than that of the zone within it. Nay, it is 

 not necessary even that each zone be made of the same kind of 

 glass. If the glass of any segment has a different refractive 

 power from the rest of it, we can make its focus coincident with 

 the rest in three ways, 1. By a slight variation of its distance 

 from the burner ; 2. By a change in the curvature of its sur- 

 face, or imperfectly by a slight variation from its proper posi- 

 tion. It can seldom be necessary to have recourse to such ex- 

 pedients ; and they are mentioned here chiefly to shew the 

 number of resources which are within our reach. 



If any segment, when finished, is imperfect, we may, without 

 replacing it, remove the imperfection in the following manner : 

 Let ABC, Plate III. Fig. 9., be a section of the segment, having 

 an air-bubble, or other impurity, as mn, then we have only to 

 cut out the portion d efg, as shewn at A'B'C', taking care to 

 make the surface ef concentric with AC, and to give the lines 

 e d, fg, the same convergency as the rays which pass through 

 that part of the segment. 



3. The construction of lenses in separate zones, enables us to 

 diminish the spherical aberration, which, as I shewed in 1811, 

 may be done by various means. 1 . Each zone may be made of 

 different kinds of glass, so as to refract the rays which they re- 

 ceive to the same focus, the radius of curvature of each zone 

 being the same. 2. Each zone, though made of the same glass, 

 and having the same curvature, may be so placed relatively to 

 each other, as to have one common focus. In Fig. 2. and Q. of 

 Plate IV., for example, if the radiating point is on the left 



