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THE PARTS OF A LANTERN 27 image. In fig. 16 let s represent the slide, a true image of which we wish to produce. Then B represents what is called the * barrel' distortion, the usual distortion of a single lens. If we correct this merely for flatness of field, which is the easiest and most obvious error to correct, we usually get the figure ewer-corrected, producing the ' hour-glass ' distortion denoted by H. Either distortion, if perceptible, is simply intolerable in architectural subjects, lines of type, or diagrams which may contain straight lines or circles. The correction of these various errors in lenses of moderate focus is a task of no little difficulty. The chromatic correc tion is a comparatively simple affair, needing simply a certain proportion, depending on the dispersions of the glasses, between the convexity of the crown and concavity of the flint; and the object of various ' figures ' for the curves, as in A, B, c, E, (fig. 14) is to correct the spherical aberrations. The forms given in fig. 14 have all been at one time or other used for photographic purposes. They all need a stop or diaphragm on the side farthest from the slide, and c is probably the best of them; but no single achromatic lens is capable of perfect correction for anything like short foci. (b) Double or triple achromatic lenses. —With foci of ten inches and over, however, the spherical aberration is much less, and these lenses then perform very well, and are in common use for long-focus work. Two or even three of such long-focus lenses combined, make better short-focus lenses than single achromatics of such short focus ; and hence it is very common to furnish a lantern with three achromatic lenses of graduated long foci, ranging from nine or ten up to eighteen or twenty inches, which by combining different pairs, or the whole three, will give fair results throughout the whole range. This result will depend upon the quality and figure of course, for of these ' triple sets,' as they are called, there are both bad, middling, and good. The only way to be sure is to have a trial, which a good optician will always afford.