5. In the burning sphere, the effect is greatly increas ed, in consequence of the shortness of the focal length of each lens, and the greater concentration of the incident light.
6. In the burning sphere, all kinds of lenses may be combined. They may be made of any kind of glass, of any diameter, and of any focal length; and the lenses belonging to different individuals may be combined for any occasional experiment, in which a great intensity of heat is requisite.
In the application of the preceding principle to a por table instrument, the different mirrors may be all ce mented together, so as to form the frustrum of a pyra mid, having as many planes as there are lenses sur rounding the central lens. A section of this pyramid is shown in Plate CVI. Fig. 6., and a plan of it in Fig. 7. Each plane glass which composes the pyramidal mirror is so placed as to reflect the incident rays upon the lens B or C which is adjacent to it ; while the central lens A converges to the same focus as the rest, the direct light of the sun which traverses the axis of the pyramid. Thus in the plan, Fig. 7., the direct light of the sun falling upon the lens A, is refracted to the focus cp, in Fig. 6.; the light which falls upon the mirror B, is reflected per pendicularly upon the lens b, which refracts it to the same focus ep; the light which falls on the mirror C, is reflected upon the lens c, and so on with the rest ; so that a whole column of rays, nearly equal in magnitude to the polygonal surface BCDEFG, Fig. 7., is concen
trated to one point cp. Instead of the glass mirrors, a conical frustrum of tin or of silvered copper might be advantageously employed.
For farther information upon burning instruments, see \Volfii Opera Mathematica, torn. ii. p. 165-167. Tra berus, In Wervo Optic. lib. ii. cap. 12. prop. 5. cor. I. Vilette, Journal des Savans, 1666, p. 125, 1st ed. p. 74, 2d ed. Mid, 1679, p. 284. P/zil. Trans. No. 6. p. 95. Do. No. 33. p. 631. Do. No. 40. p. 795. Do. 1719. vol. xxx. No. 360. p. 976. Tschirnhansen, Phil. Trans. vol. xvi. 1687, No 188, p. 352 ; and ,ict. Erudit. 1687, Jan. p. 52. Hoesen, Phil. Trans 1768, vol. lix. p. 4. La Garouste, Mem. ?cad. torn. i. p. 276. Bilffon, Sup plement a Histoire Nizturelle, torn. ii. Svo. Par. 1774. Six icme "'lemon?, p. 181-309. Dutens Du Miroir 4rdent d'./Irchinzede, Paris, 1755. 4 Description of the great Burning Glass made by M. Vilette and his two Sons, born at Lyons, with some Remarks upon the surprising and wonderful Ejfrets thereof, London, 1719. See also Ahem. ?cad. 1717. Zeiher, Nov. Com. Petrop. tom. vii. 1758, 1759, p. 237. Richman, Nov, Com. Pctrop. vol. iii. p. 340. Nollet, Mem. Acad. Par. 1757, p. 896. Kircher's Alugna Lucis ct Umbra., p. 77.2. Courtivron, Ahem.
Acad. Par. 1747, p. 659. 'Frudaine, Mem—dead. Par. 1774, p. 62. Journal Encyclopediquc, 15th Sept. 1777. Cadet and Brisson, Mem. Acad. l'ar. 1777, p. 541. I)u puy's Ahem. Acad. hist-H/11.1777. (p)