LANTERNS AND THEIR MANIPULATION n.j 

 If we want approximate focus, it will be, 



Focus = distance x slide 

 disc 



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 J 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 6-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 briefly 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 



