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. 



