. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. rthest from the slide, and c is probably thebest of them; but no single achromatic lens is capable ofperfect correction for anything like short foci. (b) Double or triple achromatic lenses.—With foci of teninches and over, however, the spherical aberration is muchless, and these lenses then perform very well, and are incommon use for long-focus work. Two or even three of suchlong-focus lenses combined, make better short-focus lensesthan single achromatics of such short focus ; and hence it i
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. rthest from the slide, and c is probably thebest of them; but no single achromatic lens is capable ofperfect correction for anything like short foci. (b) Double or triple achromatic lenses.—With foci of teninches and over, however, the spherical aberration is muchless, and these lenses then perform very well, and are incommon use for long-focus work. Two or even three of suchlong-focus lenses combined, make better short-focus lensesthan single achromatics of such short focus ; and hence it isvery common to furnish a lantern with three achromatic lenseaof graduated long foci, ranging from nine or ten up to eighteenor twenty inches, which by combining different pairs, or thewhole three, will give fair results throughout the whole result will depend upon the quality and figure of course,for of these triple sets, as they are called, there are bothbad, middling, and good. The only way to be sure is to havea trial, which a good optician will always afford. 28 OPTICAL PROJECTION. (c) Triplet achromatic^ have also been used. D in fig. 14shows a triplet devised by Dallmeyer which is said to answervery well. But a more usual form, made sometimes in France,is that shown in fig. 17, a concave of flint being used between two convex lenses as in the preceding, but thewhole lens assuming a double convex of these lenses perform exceedingly were used a great deal by Mr. Dancer forhis lanterns, and I possess a pair of them,6-inches focus, whose performance can hardlybe distinguished from that of the best of theconstruction next to be described. They appearto me to combine flatness of field, evenness andsharpness of definition, and ortho-symmetry ofimage, in a greater degree than any other single lenses, andI think it is to be desired that more attention should bedirected to this construction for long-focus work. My pair of6-inch lenses have a
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