for the purposes of Illumination in Lighthouses. 37 



" 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 spherical 

 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 corresponding 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 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 *. 



" 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. 3. the segment 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 effect. 



" 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 va- 

 rious 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 the focal point, if 

 it is thought proper." 



Although the method now described enables us to construct 

 lenses without any other limit to their magnitude, but that 



* " The burning focus lies a little beyond the red rays, and is therefore at a great- 

 er distance from the lens than the luminous focus." 



