THE PARTS OF A LANTERN 2$ 



which have been already mentioned in 4. When hand- 

 painted slides only were in use with oil lamps, two simple 

 meniscus lenses combined gave fair results, because there was 

 no detail sharp and colourless enough to manifest optical 

 defects ; but photographic slides and diagrams make con- 

 spicuous either chromatic fringes, or distortion of figure. 

 Practically, achromatic lenses only are now used in lanterns. 

 They may be of four kinds. 



(a) The simple achromatic lens, or lens composed of one 

 convex crown c, corrected by a concave of flint F, usually of 

 the plano-convex total form, as shown at A, fig. 14. TLislens 



FIG. 14. Single Achromatic Objectives 



is employed with its convex surface towards the slide s. If 

 the plane face is turned to the slide, the definition is sharper 

 in the centre of the field, but falls off rapidly at the margin ; 

 and the image is also formed on a hollow curved surface 

 instead of on a plane. By reversing the lens a little is sacri- 

 ficed at the centre, but the picture is better and more uniform 

 over the rest. A lens cannot, however, be made of this con- 

 struction of short focus, say 4J to 7 inches, which will give a 

 good image. 



It may be well to explain here the principal errors which 

 have to be corrected in the objective. As regards a condenser, 

 it has already been seen that the most noticeable result of 

 spherical aberration is to bring the marginal rays to a shorter 



