. Appleton's dictionary of machines, mechanics, engine-work, and engineering. int out the conditions which regulate its amount, as they are nearly identical with those which deter-mine the divergence of a paraboloidal mirror illuminated by a lamp in its focus. The divergence, inthe case of lenses, may be described as the angle which the flame subtends at the principal focus of thelens, the maximum of which, produced at the vertex of Fresnels great lens by the lamp of four concen-tric wicks, is about 5° 9. This will be easily seen by examining Fig. S297, in which Q q represents the lens, A its
. Appleton's dictionary of machines, mechanics, engine-work, and engineering. int out the conditions which regulate its amount, as they are nearly identical with those which deter-mine the divergence of a paraboloidal mirror illuminated by a lamp in its focus. The divergence, inthe case of lenses, may be described as the angle which the flame subtends at the principal focus of thelens, the maximum of which, produced at the vertex of Fresnels great lens by the lamp of four concen-tric wicks, is about 5° 9. This will be easily seen by examining Fig. S297, in which Q q represents the lens, A its centre, F the principal focus, bF and b F the radius of the flame; then is the angle b Kb equal to the maximum t ^ , , n- , . -^ ^F • ,, . t, rad- of flame » , , , , divergence of the lens. Sm b A i = -p-^, = sin 6 A F = ;—j—^~, ; and twice o A I< = the whole divergence at A. FQ = V(AQ2 + AF2) andQx:~FQ A F focal distance Then for the divergence at the margin of the lens, or at any other point, we have: -J (Q F2 + F x2); and for any angle at Q, we have sin F Q x. On the subject of the illuminating power of the lenses, it seems enough to say that the same generalprinciple regulates the estimate as in reflectors. Owing to the square form of the lens, however, thereis a greater difficulty in finding a mean focal distance whereby to correct our estimate of the angle sub-tended by the light, so as to equate the varying distance of the several parts of the surface; but, prac-tically, we shall not greatly err if we consider the quotient of the surface of the lens divided by the sur-face of the flame as the increased power of illumination by the use of the lens. The illuminating effectof the great lens, as measured at moderate distances, has generally been taken at 3000 Argand flames,the value of the great flame in its focus being about 16, thus giving its increasing power as nearly equalto 180. The more perfect lenses have produced a considerably greater effect. The applica
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Keywords: ., bookcentury1800, booksubjectmechanicalengineering, bookyear1861
