Bauhaus-Universität Weimar

Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Wright, Lewis
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. This lens 
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


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