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26 THE BOOK OF THE LANTERN. good form indeed, provided that the lenses are of sufficient diameter to take in the entire cone of rays from the con- denser. This has always been a stumbling-block in adjusting lenses of short focus to the lantern, for it stands to reason that the shorter the focus the nearer must the lens be to the condenser, and if the diameter of the lens be small a large proportion of the rays will not get through at all. And this question of focus of the objective is one that must be carefully considered by all who use a lantern. Many are of the opinion that the focal length of the objec- tive used should be so short that the distance of the lantern from the screen should be about the same as the diameter of that screen. In private rooms of small size this may be necessary, if not advantageous, but in larger rooms or lecture-halls a lens which will triple or quadruple that distance is desirable. Much experience of lecture- hall work has led me to the conclusion that a lens of 8-inch focus is more useful as a lantern objective than any other, and it is as well to have one of 10 inches in reserve in case the length of the hall should require it. Let me give my reasons for this choice. I find that the size of sheet most commonly required, in rooms used for lecture pur- poses is 15 feet. Some rooms will take an 18-foot sheet, and very few take a larger one than that. But the 1 5 -foot screen is the one most in request. Now let us suppose the operator has fitted to his lantern an objective of say 4|-inch focus. To cover his 15-foot screen he must plant his lantern less than 20 feet from it, a distance which will land him in the middle of the front seats. His apparatus will