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28 OPTICAL PROJECTION (c) Triplet achromatics have also been used. D in fig. 14 shows a triplet devised by Dallmeyer which is said to answer very 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 the whole lens assuming a double convex form. Some of these lenses perform exceedingly well. They were used a great deal by Mr. Dancer for his lanterns, and I possess a pair of them, 6-inches focus, whose performance can hardly be distinguished from that of the best of the construction next to be described. They appear to me to combine flatness of field, evenness and sharpness of definition, and ortho-symmetry of image, in a greater degree than any other single lenses, and I think it is to be desired that more attention should be directed to this construction for long-focus work. My pair of 6-inch lenses have a clear diameter of If inch, and require a 1-inch stop placed about 2^ inches in front to produce their best effect. In this position the stop cuts off scarcely any rays of serious importance, and the image of a slide of printed matter is exceedingly good. For lenses of 9-inches focus and upwards, no stop whatever would be required, and such lenses would be much cheaper, and pass more light than double combinations. (d) Double combination lenses are, however, most used in the best lanterns for short and moderate foci, ranging, say, from 4J to 8 inches. The type always employed is that well- known as the Petzval combination, shown in fig. 18, and in many lanterns quarter-plate photographic lenses are used for the shorter foci, and half-plate lenses for the longer. The pick of a dozen or so of such lenses will leave little to be desired; but generally a lot as imported from France by opticians is very unequal in excellence, and most of these lenses require a stop at s, which loses a great deal of light.