Bauhaus-Universität Weimar

Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Wright, Lewis
If we want approximate focus, it will be, 
Focus = 
distance x slide 
As regards foci of objectives, the most generally useful set 
of three would probably be 6 in., 10 in., and 15 in., or there¬ 
abouts. A 4^ in. lens is rarely wanted except to show through 
a transparent screen, when space is limited. The most 
useful single lens will be 6 in. or 8 in. I think myself 6 in. 
is the best upon the whole ; and if ever a G-in. lens is made 
as suggested on page 29, so that one component can be used 
alone for longer focus, as in some of the best of the photo¬ 
graphic lenses, a 6-in. lens would do a vast range of work, as 
the single focus would be about 10 or 11 inches. 
61. An Exhibitor’s Work.—We will now suppose an ex¬ 
hibitor arrived with all his apparatus in the place where he is 
to exhibit slides, with a bi-unial lantern. Let us brieflv see 
what he has to do, and how he will set about it. 
His very first step will be to get up his screen, for which 
however we will refer to the following chapter. In the next 
place he has to choose his position, as just explained, accord¬ 
ing to his focus and the size of the disc he wants to show. 
The position being found by a tape-measure, or stepping, 
the lantern will be unpacked and placed in position, generally 
on its own box (but see next chapter). Next, if it is a cold and 
damp night, and there is a fire or stove anywhere handy, it is 
wise to take out the condensers and put them to warm, and 
also the box of slides, opening the cover to let them dry. If 
gas is wanted for blow-through jets, the next step is to bring 
the supply from the most convenient place up to the lantern 
(see p. 47), and then the oxygen bag, or otherwise the pair of 
bags, are placed in the boards, or the cylinders placed securely 
where they will be used, and the regulators screwed on. 
Then we see that all taps are turned off on both the jets, 
and make the rubber connections. If the lantern has to be 


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