1. The difficulty of procuring a mass of flint glass proper for a solid lens, is in this construction completely removed.
2. If impurities exist in the glass of any of the spheri cal segments, or if an accident happens to any of them, it can be easily replaced at a very trifling expence. Hence the spherical segments may be made of glass much more pure and free from flaws and veins than the correspond ing portions of a solid lens.
3 From the spherical aberration of a convex lens, the focus of the outer portion is nearer the lens than the fo cus of the central 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 ill the present con struction, by placing the different zones in such a man ner that their foci may coincide." 4. A lens of this construction may be formed by de grees, 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 seg ment NV v n may be added to the lens without the rest of the zone to which it belongs, and it will contribute. in the proportion of its area, to increase the general ef fect.
5. If it should be thought advisable to grind the seg ments 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.
6. Each zone may have a different focal length, and may therefore be placed at different distances from tine focal point, if it is thought proper.
In the construction of burning lenses, whether they consist of one solid piece or of a number of zones and segments, a limit is necessarily put to their magnitude. As we increase the diameter of these lenses, the surface of the outer zone has a more oblique position towards the incident rays, and, consequently, a greater number of them are reflected from that surface. When this ob liquity amounts to or the whole of the incident light is reflected, and, therefore, a zone of this obliquity becomes totally useless. The diameter of burning lenses can never exceed the chord of an arch of 47° or of the sphere to which it has been ground ; and when the diameter approaches to this limit, the outer zone contri butes very little to the general effect. In order to sur mount this difficulty, we must have recourse to Cata dioptric.Instruments.