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 



