BURNING INSTRUMENTS. 



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limit is necessarily put to the in-t-niitiHf, and conse- 

 quently to tin- j'.iuer of hurmng lenses. The ex- 

 tivrm- difficulty of procuring glsss proper for a large 

 ! lens ; the trouble and expence of casting it into 

 .ticular form without faults and impurities ; the 

 t increase of ce:.tral thickness which becomes nc- 

 iry by increasing the diameter of the lens; the 

 enormous obstruction which is thus opposed to the 

 transmission of the solar rays ; and the augmented 

 aberration which dissipates the rays at the focal point ; 

 are almost insupportable obstacles to the construc- 

 tion of a solid lens of a size larger than that of Mr 

 Parker. 



Buffbn, indeed, has very ingeniously removed one 

 of these difficulties, by his proposal to construct the 

 knses of three concentric zones. The quantity of 

 glass is obviously diminished by this contrivance, and 

 the obstruction of the rays in passing through the 

 central portions of the lens is greatly lessened ; but 

 the outer circular zone must still contain as much 

 glass as the corresponding zone in the solid lens, and 

 must be nearly as difficult to cast and grind as if it 

 formed a part of the whole lens. 

 New burn- 1 order to remove these evils, and at the same 

 ing lens time to diminish the expence and simplify the con- 

 struction of dioptric burning instruments, the follow- 

 ing construction has been proposed by Dr Brewster. 

 If it be required, for example, to construct a burning 

 lens 4- feet in diameter, it should be composed of dif- 

 ferent pieces, as represented in Plate CVI. Fig. 2. 

 where A BCD is a lens of flint glass, 18 inches in 

 diameter. This lens is surrounded by several seg- 

 ments, AGID, AGEB, BELC, CLID, ground in 

 the same tool with ABCD, but so formed with re- 

 spect to their thickness at AB and GE, &c. that 

 they may exactly resemble the corresponding por- 

 tions of a solid lens. These different thicknesses can be 

 easily calculated, and there is no difficulty in giving 

 the segments their proper form. This zone, consist- 

 ing ot 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 C A 7, FBp 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 ad- 

 vantages of this construction may be very shortly 

 enumerated. 



1. The difficulty of procuring a mass of flint glass 

 proper for a solid lens, is in this construction com- 

 pletely removed. 



2. If impurities exist in the glass of any of the 

 spherical segments, or if an accident happens ti> any 

 of them, it can be easily replaced at a very trifling 

 expense. 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 central parts, and therefore the solar 



light is not concentrated in the same point of the axit. 

 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. f 



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 

 mny be u<ed as if it were complete. Thus, in Fig. 

 2. the segment NVrn may be added to the lens 

 without th-- 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 conti- 

 nuous lens. But, if it should be reckoned more ac- 

 curate to grind each zone by itself, then the various 

 segments may be easily held together by a firm ce- 

 ment. 



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. 



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, con- 

 sequently, a greater number of them are reflected 

 from that surface. When this obliquity amounts to 

 47 or 48, the whole of the incident light is re- 

 fleeted, and, therefore, a zone of this obliquity be- 

 comes totally useless. The diameter of burning 

 lenses can never exceed the chord of an arch of 47 

 or 48 of the sphere to which it has been ground ; 

 and when the diameter approaches to this limit, the 

 outer zone contributes very little to the general ef- 

 fect. In order to surmount this difficulty, we must 

 have recourse to Catadioptric Instruments. 



Burning 



Inttru- 



ON CATADIOPTRIC BUUXIN'O INSTRUMENTS. . 



In order to construct a burning instrument which 

 shall, in a great measure, be unlimited in its power, 

 we must combine the principles both of reflection 

 and refraction. We are not aware that any instru- 

 ment of this kind has ever been proposed ; and we 

 are the more surprised at this, as the proper combi- 

 nation of lenses and mirrors must naturally suggest 

 itself to any one who considers the limits which are 

 set to the construction of single lenses, ar.d the dis- 

 advantages, either of a theoretical or a practical na- 

 ture, to which they are liable. 



Thii instrument, which has been proposed by Dr 

 Brewster, and which may be properly called a burn- 

 ing sphere, from the arrangement of the lenses, is re- 

 presented in Plat'- CVI. rig. .". which is merely a 

 section of the sphere, and represents only fire of the 

 lenses, and four of the mirrors. The lenses A, B. 

 C, D, E, which may be of any diameter and focal 

 length, are so placed in the spherical surface AMN T , 

 that their principal foci exactly coincide in the point 

 F. If any of the lenses have a different focal length 



On catadi- 

 optric 



burning in- 

 strument*. 



Burning 

 sphere pro- 

 posed by 

 Dr Bre v - 



StCf. 



PLATE 

 ( VI. 



f The burning focus lies a little beyond the red ray*, and is therefore at a greater diNtur.ee from the lens than the luniiuout 

 focus. v 



