. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. , it would be better that the lens nextthe lamp should be rather deeper in curve, but the great heatmakes this undesirable. With the lime-light it is different,and in considering the very best form, it is well to understandthe principles upon which the correction of aberrationsdepends. This has been very familiarly explained in thediagram, fig. 11, by Bow. Here d e re-presents the upper half ofa plano-convex lens, thefaint line he the outlineof a double convex lens incontact with i
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. , it would be better that the lens nextthe lamp should be rather deeper in curve, but the great heatmakes this undesirable. With the lime-light it is different,and in considering the very best form, it is well to understandthe principles upon which the correction of aberrationsdepends. This has been very familiarly explained in thediagram, fig. 11, by Bow. Here d e re-presents the upper half ofa plano-convex lens, thefaint line he the outlineof a double convex lens incontact with it, and de ameniscus lens of the samefocus. Consider now thedifferent effect of these twolatter upon the spherical aberrations, due to the fact thatthe marginal rays are brought to a shorter or closer focusthan the central rays. It is quite plain that the focus p ofthe central rays, a a, will be almost exactly the same inthe two arrangements—we may practically consider that afixed point. But with the marginal rays, bb, it is the top one, b d, let us suppose that the lens h re- 02. Fig. 11 OPTICAL PROJECTION fracts it to/2: then the distance between p and /2 repre-sents the aberration of the d h combination. But, owingto the curvature, away from the lens d, of the meniscus d,the marginal ray passes through d nearer the centre thanthrough h, and consequently its second refraction by such alens is less on that account; the same ray also passes throughthe meniscus at a less angle of incidence, which in anotherway also reduces the second refraction. Consequently, themarginal focus is lengthened, and the aberration is reducedto the distance from f to /. The main factor in this correction is the bending awayfrom each other at the margins of the two lenses, which isobtained equally in the double piano form, and explains its superiority to the two doubleconvex lenses, c, fig. 10. Butit will be evident that the othercondition, of minimum devia-tion at the margin, is on
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Keywords: ., bookauthorwrightle, bookcentury1900, bookdecade1900, bookyear1906
