166 Dr Brewster on the Construction of' Polyzonal Lenses 
It has been already stated in the preceding extract, that the 
spherical aberration of the lens may be nearly corrected by a 
proper adjustment of the zones. By examining Fig. 3., indeed, 
it will be obvious, that a lens constructed exactly like it, will 
have less spherical aberration than if the lens were in a solid 
state, provided the parallel rays are incident on the continuous 
face ADEB. For since the focus of the central portion is al- 
ways more remote from the lens than the focus of the contiguous 
zone, the focus of the thin lens DE, will be nearer the lens 
AB than the focus of the corresponding portion D £ £ E in the 
solid lens, and consequently the breadth of the zones may be so 
adjusted that the mean focus of each shall coincide in the axis 
of the lens without displacing them from their virtual position 
in the solid lens, that is, without breaking the; continuity ACDEF 
of their common surface. This curious property of diminish- 
ing the spherical aberration, does not belong to the method of 
forming the steps adopted by Buffon ; and M. Fresnel, in whose 
lenses the steps have the same position as in those of Buffon, 
seems also to have overlooked it. 
If good and colourless flint-glass could be procured for the 
segments of each zone, the lens might be built without steps, 
or of the solid form A mp B, if solidity should be considered 
requisite, when its magnitude is to be very great ; though even 
in this case sufficient strength might be obtained, without resort- 
ing to such an alternative. 
The spherical aberration in polyzonal lenses might also be 
removed by using zones of different refractive powers, the densest 
being placed in the centre, thus imitating in a coarse manner the 
crystalline lenses of animals ; or the same effect might be pro- 
duced by a slight increase in the focal length of the zones as 
they recede from the centre. 
In all these cases, however, the directions of the bases m C, 
n D, o E and p F of the zones, should converge to the focus of 
rays refracted at the first surface of the lens. 
The principles which have now been described in reference to 
the formation of lenses, are equally applicable to Polyzonal Mir- 
rors of a large size, which may be constructed of any magni- 
tude, and the successive zones so adjusted as to produce a per- 
fect coincidence of the mean focus of each zone. 
-In my Treatise on Nczv Philosophical Instruments (p. 399 
