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. 



