169 Dr Brewster on the Construction of Polyzonal Lenses 
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 direction 
of the radius. The section of this lens is represented in Fig. 3. 
where DE is the central portion, DC EoF the second zone, 
and CAm, FRy? 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 enumerated. 
44 1. The difficulty of procuring a mass of flint glass proper 
for a solid lens, is in this construction completely removed. 
9. 44 If impurities exist in the glass of any of the spherical seg- 
ments, or if an accident happens to any of them, it can be easily 
replaced at a very trifling expence. Hence the spherical seg- 
ments may be made of glass much more pure and free from 
flaws and veins than the corresponding portions of a solid lens. 
44 3. From the spherical aberration of a convex lens, the focus 
of the outer portion is nearer the lens than the focus of the cen- 
tral parts, and therefore the solar light is not concentrated in 
the same point of the axis. This evil may, in a great measure, 
be removed in the present construction, by placing the different 
zones in such a manner that their foci may coincide. 
44 4. A lens of this construction may be formed by degrees, 
according to the convenience and means of the artist. One 
zone, or even one segment, may be added after another, and, at 
every step, the instrument may be used as if it were complete. 
Thus, in Fig. 2. the segment NV v n may be added to the lens, 
without the rest of the zone to which it belongs, and it will contri- 
bute, in the proportion of its area, to increase the general effect. 
44 5. If it should be thought advisable to grind the segments 
separately, or two by two, a much smaller tool will be necessary 
than if they formed one continuous lens. But, if it should be 
reckoned more accurate to grind each zone by itself, then the 
various segments may be easily held together by a firm cement. 
44 6. Each zone may have a different focal length, and may 
therefore be placed at different distances from the focal point, if 
it is thought proper.'” 
